Wow, great info so far. I was going to ask a follow-up question of whether I should build the lower and upper or just the lower, but it sounds like I have my answer. My intention now is to purchase a billet upper and a bolt assembly and send it all to Krieger to have them do the install. They did an M1A for me a while back and I was very happy with it. I'll be using their handguard and gas tube. If I can get a .204 barrel, I'd go with the 1:12 twist, since I pretty much only shoot 32 or 35 grain bullets. If I can't (see below) I may either go with .223 or try a different barrel manufacturer.
As for the trigger, I did a bunch of research last night after reading TonyAngel's post. Normally I wouldn't post things from other forums, but I found a good comparison of AR triggers here:
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=501733. I was going to try living with a stock trigger but now I've changed my mind. I think I'm going to go with the Geissele trigger, either the SSA or the Match.
Here's my next batch of questions.
1) Who makes billet uppers? I haven't seen many other than the VLTOR MUR.
2) Should I care about a forward assist? It seems like it wouldn't be a big deal to not have one on a bench gun.
3) People have mentioned getting a matched bolt and barrel. Sending in my bolt and having a custom barrel made is the same thing right?
4) I've heard about changes in charging handle design lately. Is something like the BCM Gunfighter handle useful on a bench gun?
(A note on Kreiger AR barrels in .204)
I emailed Krieger yesterday and asked if they could do one for me. Their response was that they do not do individual orders on these because of the difficult set-up. They do make small production runs for different gunsmiths and when they do they usually make a few extra, so I may get lucky and catch one.