I just ordered a 20" stealth barrel from Larue with gas block and gas tube. What upper and handguard would you recommend to complete the package?

I suggest checking out the precision rifles section. Those guys get a lot more in depth with their research.

http://www.ar15.com/forums/b/16_Precision_Rifles.html

im in the middle of a 20" precision build.  Im using a CMT Tac UPUR-1 and a 16.7 NSR.  

Link to my build thread

I really enjoy my Geissele MK1 on my SPR.  Super rigid.

I think a MUR or UPUR would be a great starting point for any precision gun.

Perhaps a KAC URX4 MLOK, or FF-RAS Rifle, or URX3.1 13.5 or even a rifle length URX2 or 3.

The Noveske NSRs are also very nice.

Even an ALG EMR V3 would be a good choice in my book.

Geissele MK4 15" for your 20" rifle gas barrel.  Here's my 16" Stealth barrel on the Geissele MK4 13" rail.  I used a standard forged upper, but there's other options out there if you want to spend the money.  Vltor MUR, Larue, etc.

I really think that would come down to personal preference. If you want to get really into it, you could look at lapping the mating surface on the upper to be truly square to the barrel extension, and if you have slop between the upper and extension, you could shim it or bed it. Other than those things, I don't think there's very much going on with the upper receiver or free float tube that really affect function or accuracy.

The fancy machined billet stuff may look better to some folks, but I really think it's mostly cosmetic. Of course, I could be totally wrong.

My taste leans toward a forged upper minus the forward assist. Aero makes one with the dust cover and brass deflector, but no forward assist. I really like that look, and I've never needed the forward assist in all my years of shooting either in the military or out. It's a cleaner look with a minor weight savings too.

Find the parts you like to look at, and go with them.