Just play a straight Eldritch archer, and perhaps go for the musket instead of a pistol. Mage Bullets would be almost worthless for this build as you already are targeting touch AC most of the time and cast boost your attack via arcana or arcane strike. As soon as you can put Distance on your musket (80' range for touch AC), and use your Magus Arcana for whatever boosts you need (flaming, etc). Buy some deliquescent Gloves and all your attacks already have Acid. Pick up Ranged Spell combat and Reach Spellstrike. use your spells mostly for things like dimension door, phase step, haste, or shield. but have a few shocking grasps, lightning bolts, umbral strike, and cosmic ray type spells.
Be sure you have Rapid Reload and Alchemical cartridges (Paper).
1. Can I sacrifice my magus spells for Mage bullets or I'm limited to my wizard spells?
Yes, although both your Magus arcane pool ability and the Mage bullets ability are limited by the +10 maximum enhancement bonus limitation, so at higher levels they will become redundant. At lower levels, the duration is a bit short for what it does, but it can be a nice way to boost damage output. Remember that you want special abilities, not just plain enhancement bonus (you have better ways of getting a simple attack buff).
2. If I cast a Spell with Ranged spellstrike can I add the Mage Gun bonuses to my magus spells?
Yes, you can use these together. The Mage Gun ability can be used whenever you cast a spell, and the Spellstrike is still casting a spell. Both of them require the spell to be cast through a weapon, but it's the same weapon so that's fine.
For example, I cast fireball, which I learned as a magus, if my Gun is +1 can I add this bonus to the dc of the Spell?
No, Fireball is not a legal choice of spell for either Ranged Spellstrike or the Mage Gun. You will need to use a ranged touch attack spell such as Scorching Ray or Battering Blast if you wish to use both.
I would also caution not to use area of effect spells with the mage gun. The chance of an overload is really high if you use area of effect spells, and you do not want that.
One feat I would highly recommend for a build like this is Spell Cartridges. This allows you to reload as a swift action, and causes your attacks to deal force damage. This gets around reloading issues (though you can still reload normally if you need your swift action for other purposes) and also means you ignore DR. It's a really strong and underrated feat; people are right to question its requirement of using a swift action every round to activate it, but it's totally worth it. I saw a similar Magus build use it in my last campaign, and it was really dominant. It was like a Paladin, except no daily limits on smites and it also worked against golems.