I think you're better off getting an 18 Strength and a 14 Charisma and fiddle with your stats some more. Maybe up Con to 14? or Con 13 Dex 12? Or Int 13 somehow and qualify for Unsanctioned Knowledge which is pretty awesome.
10 Int, human paladin and favored class benefit (hp) would indeed get 3 skill points per level.
Don't bother multiclassing. Paladins get too nice stuff to delay it.
For thos variant class features (called archetypes), the most popular one IMO is Oath of Vengeance. Gets extra spells added to spell list and instead of channel energy, can expend 2 lay on hands for an extra smite.
Feats: Fey Foundling from Inner Sea World Guide and Power Attack are excellent level 1 feats for a human paladin. Fey Foundling works REALLY well with lay on hands.
Unsanctioned Knowledge is good too. Might need the Magical Knack trait in order to get a worthwhile caster level for whatever spells you pick.
Extra lay on hands, level 3 and onwards is good.
If you're keeping the Cha 16 and upping it each 4th level, consider Greater Mercy and Ultimate Mercy for free raise dead effect when you ever get at least 10 lay on hands per day.
If you don't have such a high Charisma cuz you preferred Strength instead, Channeled Revival is another way to revive party member, but has to be within 1 round of the PC's death, but still has the channel energy range of 30 feet, so still useful.
Spells:
I only looked at levels 1 and 2, but:
1: Hero's Defiance and Grace
2: Litany of Righteousness and Paladin's Sacrifice
..are good picks to front-line (tough and mobile) and keep yourself and others alive.
Human's a great choice. Bonus feat is excellent. I'm neutral about the weapon. Paladin's typically want a great threat range for smite to be miltiplied on a crit, such as a falchion/nodachi/scimitar, but the heavy flail is a good cheap weapon for low levels and you can always switch out to different weapons if you aren't married to the concept of flailing.
Other good race choices: Half-elf, due to recent FAQ ruling, half-elf counts as both human and elf for alternative racial favored class benefits, racial feats, and that means you can get the elf paladin favored class benefit each level: Add +1/2 hit point to the paladin's lay on hands ability (whether using it to heal or harm). So the best advantage of elf paladin without the -2 Con penalty.
Tieflinghas: Add +1 to the amount of damage the paladin heals with lay on hands, but only when the paladin uses that ability on herself.
Also the demon-spawn tiefling (scroll down in page) has good racial stats for it.
This is not a Beginner box question, it should be moved to the appropiated thread.
Happy rolls
If you're wearing heavy armor, high Dexterity is not necessary (unless you're a fighter). The max-dex stat on heavy armors cuts down on its usefulness. As for Constitution, I'm playing a character in a game right now with d8 HD and +1 from favored class bonuses each level, but stuck with 10 con. The result? At level 6, I have under 30 health and nearly get killed every game. Trust me, you want that extra HP.
Oh, and to answer your question, level 1 HD is maxed.
Ability Scores
People don't recommend anything less than 12 Constitution for melee characters due to needing hit points, and because in Pathfinder, one's usually dead hit points are at negative Con score. Assuming you stick with a 13 Con and favored class benefit (hit points), your starting HP is 12 (Yes first level hit die is maxed out for PC classes) or 15 with Toughnessf feat which means couple hits could still bring you down as fast as any fighter or ranger who all benefit from a not-terrible Con. Level 2 with Lay on Hands makes this tons easier.
Dexterity isn't that useful for paladins. At most they'd ever usually need is 12 in order to benefit from fullplate (unless it's mithral of course). I'm not sure how much of the rules you know of Pathfinder over 4E, but all the armor has a Max Dexterity bonus, which limits how much of one's Dex mod benefits AC, the heavier the armor the lower the Max Dex bonus. Dexterity skills are rather limited for your paladin. You stated you don't want a mount, so Ride (the only Dex class skill) is out. Acrobatics/Disable Device/Sleight of Hand/Stealth aren't class skills, and even if you made one of them a class skill with a trait, you'd still suffer armor check penalty; if they aren't class skills, the armor check penalty would hurt even more. Dex is useful for initiative, sure, but hardly seems like a fair tradeoff for less hit points; and if you're limited to Core-only feats Improved Initiative is looking pretty good.
So people don't consider Con to be most-definite third most important stat, it just gets such acclaim only be default since Dex, Int, and Wis aren't seen as useful as more hit points usually.
If this was an Archer Paladin, completely different story. Up Dex all the way!
Weapon
The reason why you hardly ever see Improved Crit + Crit Focus with a heavy flail is simple. There are plenty more weapons that take advantage of those feats better than the heavy flail due to 18-20 threat range: martial options include scimitar/falchion/nodachi. It's understandable if you want to use the heavy flail for flavor or in-character reasons, but everyone would consider a falchion's 2d4 15-20/x2 (30% chance for critical threat) damage is stronger than heavy flail's 1d10 17-20/x2 damage (20% chance for critical threat). Even a greatsword has more damage with the same threat range. And when you're on the forums, folks use the more optimized examples to make their points and I'm afraid the flails don't get a lot of love.
With a paladin's smite damage combined 2-handed power attack, the damage modifier is quite significant, especially at higher levels. Criticaling more often and doubling all that high damage modifer from 1.5xStr + smite + 2H power attack is seen as the best way to take advantage of it.
Feats
Due to your DM/group's severe ruling of core rulebook-only feats, your choices are very limited. Your list of Power Attack, Toughness, Weapon Focus, and Extra Lay on Hands is fine due to lack of anything better. Though I think Extra Lay on Hands is considered better than Toughness since you can self-heal as a swift action and it powers your Warrior of the Holy Light archetype abilities. But if you get Toughness, it's better to do it early or not at all.
Also, I feel bad for anyone thinking of playing barbarian without Raging Vitality feat or a monk without access to combat style feats in your group.
Is it correct that with toughness, a +1 Con mod and favored class level into HP you get 3 extra HP on 4th lvl and thereafter?
Assuming you're asking if you'd get 1d10+3 every lever from level 4 onwards, then yup.
About Improved Crit + Crit Focus I actually ment why those two feats themself dont seem so popular, independend of the weapon. Of course it should be one with a good critrange. I almost couldnt believe that you can get a 15-20 crit range with a Falchion, but that was probably because of my DnD 4e experience. Since in 4e you dont have to confirm a Crit.
I think a lot of people think Critical Focus is usually only worthwhile if you're also picking up the Critical feats, like Staggering Critical or Stunning Critical, and typically recommended to fighters due to their bonus feats. A +4 critical confirmation on its own for a feat seems kind of low for a full BAB class with good strength. But then again, you're wanting to play a Warrior of the Holy Light, so you get less self-buffing opportunities, so maybe it's worth it.
Thought I d get a beating for choosing the Warrior of the holy light, critics werent so good after the errata...
Meh people should play what they want to play. People on boards generally just want to make sure everyone's aware of all the ups-and-downsides of everything so a player can make an informed decision. You read the comments on the archetype post-FAQ so no need to get into it. After ALL the negatives, and you'd still wanna play it, more power to you.
And our limitation on the core rulebook isnt really forced. We just dont want to overhwelm the new ones to DnD and the others are atleast new to Pathfinder.
But if I am missing out on something super great for my Paladin I could probably get that feat too.
Well maybe check out Furious Focus feat. And definitely look over Fey Foundling again if your GM might be good with it.
Fey Foundling healing with lay on hands:- 2nd level: 1d6+2
- 4th level: 2d6+4
- 6th level: 3d6+6
- 8th level: 4d6+8
- 10th level: 5d6+10
Also we got two undecided ones. And we need another front line guy. And one of them wants to play a damage dealer.
The completly undecided one always played fighter typ calsses and wants something different now, better not to complicated. Ideas?
Barbarian is a good choice. Tough and high damage. Invulnerable Rager is popular for DR/- = to half barbarian level. But would need Raging Vitality feat from the APG in order to not simply die whenever he falls unconscious. For example, if a level 3 barbarian with 16 Con doesn't have that feat, and was raging (+4 Str, +4 Con) and was brought down to -10 HP, he'd lose rage for being unconscious by default:
If a barbarian falls unconscious, her rage immediately ends, placing her in peril of death.
and then he'd immediately lose 6 HP because he'd lost the +4 Con from rage which normally gives him +6 HP (2 per level). His HP would drop down to -16 and he'd be dead. Raging Vitality would allow them to keep raging while unconscious, while this may seem like a waste of rage rounds, keeps the barbarian alive long enough for emergency healing.
So a player wants to play something besides fighter (most straightforward class) but no too complicated? Hmmm...Ranger? Some spells, sure but that can be swapped out with specific archetypes if he doesn't want it. Got to make smart choices with favored enemy though. But besides that, loads of skill points and full BAB would make the character easy to play (less opportunity cost trying to pick between skillful options or combat options). The extra combat styles from APG should also be checked out as they can bring on entirely differently flavor of ranger for combat.