Well the title states is all.  Never really messed with handguns besides a good cleaning and will be buying parts for my first glock.  I got a PF45 polymer 80 frame on the way and plan to build a G20 gen 3.  I guess I will ask for a Tier 1 parts list but like triggers for AR's I would buy LaRue before doubling over for a Geisselle.  In this case I might buy the Geisselle but I do like options.  I probably will run an RMR on the slide but it might be a minute before I get that far.  This will probably be a home defense gun or rang toy, doubt I would do EDC with it but I wouldn't count that out in today's world.  

Any pitfalls of buying the wrong parts would be appreciated, such as it appears a gen 4 trigger is not the best option.  Not sure what generations of different things are compatible or not.

You will definitely want to use a Gen 4 G21/21SF trigger housing with the Polymer 80 PF45 frame. A Gen 3 trigger housing will not work.

The Marine Gun Builder forum is a great place to get help building Polymer 80 frames and the web site has great videos to help.

Marine Gun Builder

There is also a great video explaining how to fix the Gen 4 trigger housing to fit properly into the PF45 frame.

Yup, +1 on Marine Gun Builder.  Videos are a little long-winded, but great.

Thanks....off to go research.

Get all Glock OEM parts. No fancy gold barrels, etc. For the trigger, just lightly deburr metal-to-metal contact points (if necessary) and polish metal-to-metal contact points (bird's beak, connector, cruciform, shark fin, safety plunger, top/rear of cruciform and striker sear.

Run it and if you feel like you need to change something, pick up a minus connector and a Johnny Glocks trigger shoe (it is Glock OEM and has a smooth face, shaved trigger safety and pre-travel adjustment).

I am looking at purchasing a few mags, will regular 15 round Gen 4 G20 mags work?

Yes Gen 4 magazines work just fine in the PF45 frames. My OEM G21 magazines are a mix of Gen 3 and 4.