Advice on filling a campaign

[Insane_Unicorn]

Hey guys,

I'm about to launch my second campaign as a GM with a group of three players I have played with for a few years now.
What I want to do is give you an overview about the story im planning and you to help me create some details and help preventing common mistakes in the settings.
My main inspiration is the book "Into the darklands" http://paizo.com/products/btpy85ej?Pathfinder-Chronicles-Into-the-Darklands .
So here's what I'm planning to do, roughly split into four arcs:
Arc 1 (Level 8-10):
Players start at around level 8 in a larger city with the goal to uncover a smuggler/drug ring. So the first focus will be some investigations, few fights with human enemies (mainly rogues of course ) and just see how the party tends to handle things. Eventually they will find the laboratory in which the drugs are made and be exposed to the drug itself, Midnight Milk https://pathfinderwiki.com/wiki/Midnight_milk . Here's the first mini boss in the form of an alchemist (say hello to professor putricide from icecrown citadel xD ).
One of the players will coorporate with me from there on as a secret intellect devourer who has taken over one of the bodies and will try to lead the party to Ilvarandin to take over the other party members (the player already knows that and is looking forward to it). https://pathfinderwiki.com/wiki/Ilvarandin .
Arc 2 (Levels 10-12):
Will be the search for an entrance to the darklands, probably to Sekamina and the descend into it. I plan to give hints on like three possible choices to present to the players and let them choose which is why i can't go into details yet.
Arc 3(Level 12-?):
The way through sekamina to Orv. The darklands itself provide a lot of challenges and depending on how well the the campaign is received this will be more or less spiced up with optional quests and exploring since the party isn't really in a rush. As far as i know my players they will greet any opportunity to explore lost places if you lure them with powerful artifacts which i intend to give them at this point. If you got any interesting one-shots or similiar that would fit at this point i would be very happy to hear them.
Final Arc:
Getting to Orv/Ilvarandin and find out they have been tricked. "Winning" on their own this point against a whole city of intellect devourers seems unlikely so i need some other reward for the party. Finding and rescuing the mind of the other party member? Destroying a powerful artifact as revenge? I also thought about giving them the chance to meet neothelids, who are at war with the intellect devourers and help them.

Last but not least: What books would you suggest i allow? I kind of want to allow the psionics books since they fit very well in the settings but i have no experience with them and two of my players are real munchkins.
Let me know what you think about the story, mistakes i should avoid and any cool ideas you got :)

[Asmodeus' Advocate]

My go to for allowed materials is, everything goes but it's your job to balance your character, not mine. If it ruins the fun for other players, it'll be banned or nerfed when it comes up. I find that this is a nice way to let people have whatever concept they want without leading to inevitable munchkinning.

But then, this likely wouldn't work for everybody. With the campaign starting at level eight you can't rebalance with gear drops, either. So if you don't feel comfortable allowing something, don't.

As far as general advice goes, the best advice I give tend to be links to other parts of the internet. Here's one right now!