I think an inkling of what you're looking for in this rifle would be handy in narrowing down some choices. Varmint, Home Defense, smokin skinnies, action shootings, long range?
For any serious (HD/combat) application, there's a short list of companies you'll want to consider:
Noveske
LMT
Bravo Company
Spikes
Daniel Defense
Colt
Most of the other brand names, like DPMS, Stag, Bushy, Model1 will do you fine for non-essential uses (varmint, hunting, light competition etc).
Personally, I run a Colt HBAR 20" A2 upper. It has a 1-7" twist, which is as fast as you'll normally find, and which is also current mil-spec IIRC. 1-7 is appropriate for basically the entire gamut of loadings available in 556/223. Slower twist rates will generally not stabilize heavier projectiles.
In general, 16" is about 'standard' for a carbine-type rifle. 16" barrels are available with carbine and midlength gas systems. The carbine-length system is easier to find, and get handguards and shit for. The midlength is generally considered to run more smoothly with the 16" barrel. I've found guns with 16" barrels to be rather handy sized and typically balance in a way that works for most people.
18"-20" barrels will give you a full-size rifle. The Navy SPR and MC SAM-R run 18" and 20" barrels, respectively. Most combat applications of these length barrels will be designated marksmen-type roles. If you wanted to build a retro A1 or A2, 20" would be accurate.
24" barrels are usually found on varminter type guns. It's ALOT of barrel.
As for DI/Piston....DI works, if you take reasonable care of the rifle (oiled, clean it every now and again, keep as much dirt/sand out as possible) . Piston works too, but is typically a lot more expensive, and for the average user....not necessarily worth the cost.
As for triggers....I use a standard trigger. The RRA 2-stage, from what I've seen, is popular. Timney and Giessele (sp?) are pretty popular as well. Someone else will have to speak about the specifics of these triggers, though.