Quoted:
do you like shilen barrels? how critical is the bolt and carrier to accuracy? how big of a difference between say, 18 and 20 inch. How big of a difference does the profile of the barrel make, if any. do bull barrels affect accuracy? is it possible to have a piston system on a match barrel? will that affect accuracy?
I may be offbase due to my limited experience but I'm not a believer in bolt carrier groups doing much. My "precision build" uses just a stock rock river arms bcg. Some will swear by a low mass bcg or some special coated but not I. Accuracy of a barrel has nothing to do with length. At distance is where you will see a 24" beat an 18" but inside even 400 yards, it is negligible in my opinion. At range, you will see the slower bullets dropping more and be effected by wind more. A bull barrel will just simply help with your heating. It won't get hott as fast but it also won't cool fast. If you are shooting under 300, I'd feel fine buying a 16" but if you are like me and shoot it to 600, I'd go long.
One major aspect I feel you have overlooked would be the twist weight and the gas block. Personally, I like a 1/8 twist. It will stabilize anything you are trying to load mag length. As far as the gas block goes, get an adjustable. I like mine to where it just barely cycles with the match load. This gives less wear on moving parts and helps you spot your hits. It will make the 3 recoil pulses feel like 1(fires projectile)...2 (bcg hitting rear... 3 bcg slamming home instead of 1 2 3....
I'd spend my money on a barrel, trigger, stock and bipod. You will only shoot as good as you are comfortable behind the gun and a good smooth trigger will help you with the pull. As far as the stock goes, If an A2 gives a good LOP I'd just duct tape a small piece of foam to the stock at a height perfect for your cheek weld. Unless multiple people will be using this gun, no need for an adjustable stock like a PRS. Just use tape to make your cheek rest.
My rig is a no name lower, DPMS LPK, A2 stock with a RRA 2 stage sporter. Upper consists of a no name upper, RRA BCG, YHM free float specter length rail, low pro gas block with a wilson 15.6" stainless 1/8 twist barrel. I built it on a college budget. If I were to do it again I'd use the adjustable gas block and a 22-24" tube since I shoot it out to 600.
Sorry for the book of a response.