I got hooked on building my own AR's about 2 years and have built 3 for myself and several for frineds. What I really like about it is you can pick and choose what you want vs. buying an existing retail package; typically much less than the retail package. For example I put togeather a rifle which is very, very, similar to the RRA Coyote (MSRP $1550) for a buddy and by the time we were done he had about $700 in the gun.
A few suggestions I would like to make in hopes you can learn from my mistakes.
Spend a little money on some necessary tools up front:
1) Roll pin and punch set (Brownells is where I bought mine)
2) Get a good multi pupose Barrel Nut wrench. I have the DPMS version which I purchased from Brownells
3) A few years ago Colt came out with a change to their upper and barrels where two of the bolt lug grooves extend into the upper. From my understanding this was due to possbility of a jam in extreme cold weather conditions (I suspect someone will correct on this but it's what I was told by a gun smith). The reason I point this out is not all uppers are groved to accomodate this type of barrel. For the life of me I can't remember the term, but you can clearly tell the two uppers apart when you look at them.
4) Buy a good receiver vise. Brownells carries one which encases the upper, while DPMS sells one that slides into the upper and is connected via the take down pin holes (personally I like the DPM version because it will accept any AR-15 upper)
5) Download the USMC AR-15 barrel assembly instructions. 3 times to 30 ft lbs. and then no more than 80 ftlbs when lining up the gas tube hole.
Before you buy any upper or lower kit, decide if it would be better to mix and match. i.e. maybe a two stage or match trigger vs. the single stage found in the stand lower kit. This is also time to think about your trigger guard, I would suggest going with the cold weather one as you have a little more room.
Look at the different stocks available. Personally I like the mag-pull because it has an adjustable comb and stock extension.
Do you want to affix any after market goodies, lights, bi-pod, etc. If so you will probably want to go with a quad rail system in place of a standard free float; Sampson and LaRue make excellent ones (albeit pricey).
Little things most people don't think about until after the fact would be adding a baddger release to your charging handle (sure makes gripping it much easier with a scope mounted). Which grip to put on, I like the DPMS sniper. Think about your optics before you build the rifle.
There are alot of things to think about, but if you take your time and do your research you can build one heck of rifle. And, don't worry about using differnt mfgs for different parts of the rifle. The only one of 5 I have which is 100% pure is my DPMS 24" varmiter.
Just my two cents
Cheers-
