1. I need to finish out the lower receiver, where and what should I buy? Is a drop in trigger housing good or just easy? I like a good trigger but don't need anything too fancy or light. Something close to my Kimber would be ideal.
Best bet would be to pick up a DPMS lower parts kit (LPK) which you can get new in package on Gunbroker for $50 (the guy I bought mine from swapped in a Magpul winter trigger guard for little cost)
2. Should I get a complete upper or buy the parts separately? I want a flat top with carrying handle (I'll add optics at a later date). I'd like a good barrel but don't need a super duper tack driver. What is a good barrel when it comes to quality vs. accuracy. I think I want chrome lined.
Get the complete upper, I did mine myself with some help from on-line sources, and it wasn't THAT hard but unless you get a real deal on each part you're better off with a complete upper. As for who, I'd highly recommend Stag, their upper and lower fit is nice and tight, if you add on another maker you may have a slightly loose fit between the upper and lower and need to buy these little spacers. If you want to see where to buy one, you can browse the dealers on AR15.com and they are usually reputable folks to work with. If you think you want Chrome Lined, get it. If you are only going to the range with your rifle you may not need it.
3. I'd like the foregrip to give me the option to add a light in the future.
Way to many options, and they are pretty easy to swap if you don't like the stock ones that come on the upper you buy.
4. I think I'll go with a collapsable stock. Are they all about the same or do I need to beware of certain brands?
Again, pretty good options out there, nothing to really avoid. Keep in mind any local laws... ie in NY I can only use a fixed A2 style or a pinned A4 (or a stubby A2 which is pretty sweet).
5. I believe I've settled on a 16" barrel to give me more indoor maneuverability. What am I giving up accuracy wise if I get a 16 incher vs. a 20 incher?
Not much, but that depends on what you will use the rifle for. I planned mine out for 3-gun style shooting so I went with a 16" lightweight Colt barrel in 5.56 NATO. If you want to go long range varmint you would want a heavy long barrel with a lightweight trigger to maximize accuracy.
Good luck with your build and welcome to the world of BRD (Black Rifle Disease).
Here's what my build ended up like (for the time being.)