There are so many options it's hard for anyone to recommend just one. LIke Skyrr has linked, I too would get a better trigger than a stock one, I prefer good 2 stage performance/competition triggers in all my AR platform rifles. So far as the uppers, here in Canada all are lengths are in the same class - ie no short barrel rifle sbr NFA stuff - so I have usually gone with shorter uppers with my 80 percent builds in the past, as they have been cheaper, and the whole point of doing an 80% IMO is to try and build a low cost rifle for the kicks. Too each their own in that dept. I have two 80 percent built on Dlask lowers, one is a 10" DPMS upper, the other is a 7" DPMS. Very large muzzle flash from the 7", and very low velocity too, but it's just a range/collection rifle, I barely shoot it.
For the $ the smoothed out ALG triggers are tough to beat IMO. I've spent upwards of 300$ on triggers before, but on an 80% this is overkill in the extreme IMO, and ACT for 65$ isn't a huge price to pay for what you get out of it. If price isn't a huge object, IMO the best bang/$ for a 2 stage performance trigger is RRA's 120$ 2 stage competition trigger group. It's adjustable from 3 to around 5 lbs, and is a pretty glass trigger for the price IMO, I have one in my 20" heavy stainless AR target rifle, and I like it. It's not quite as good as the Gieselles, but is less than 1/2 the price or even cheaper too.
Post some pics of your build, it's always interesting seeing who does what with their AR platform projects. One thing that isn't so common anymore is doing a full length 20" barrel with a regular A2 stock. There are advantages to this setup, especially for all round tasks and longer range shooting, if you're doing that sort of thing - the added velocity at 500 yard stages in comp or varmitting/etc can't be discounted with the 20" barrel, and the stability and strength of the A2 stock, as well as the simplicity and constant length, isn't always a bad thing either. Just an idea, I have an original Colts A2 Hbar AR15 that is still unfired, so my next AR platform rifle will likely be something along these lines, or possibly a USMC A4 copy type of thing.
THere are ALL kinds of AR uppers on the net, gunbroker and the like. Considering you're in the USA (I think), if you want to avoid the SBR/NFA stuff and get the 16.1 or whatever you have to do, for 250 to 450$ there are piles and piles of options, all good IMO. http://www.gunbroker.com/Auction/ViewItem.aspx?Item=455116122#PIC
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=455087653#PICAnd so on. Some come with the bolt carrier group, some don't. The DPMS ones for mid 300$s usually do. You can easily build a rifle under 750$ with sights, stocks, lower completion kit/trigger group, and all that now. Remember when the "scare" was on and you couldn't find one for less than 2k, even the crap?
I guess the type of ammo you plan on shooting, and the type of shooting you plan on doing mostly will dictate also the twist rate and type of upper as well, so take that into consideration too. If you're only going to be shooting less expensive 55 gr ammunition, get a twist rate that is more compatible with that, same goes if you're mostly shooting 62 gr or higher like the 262 round - get the twist that works best with that, and so on. 1:9 is common, and great for 55 gr, and you can even sort of shoot 62 rnds (not optimal) and get ok results, but if you're mostly shooting surplus military 62gr like we get here in Canada, I would try for a 1:7 upper, which will also shoot heavier gr bullets well. It is an important concern and issue, so read the upper description closely with all adds. Last but not least, have you decided if you want a direct inpinge system, or a newer piston type upper? There are pros and cons to both, and cost comes into that is well IMO.
