I am going to start to shoot 3 gun matches next summer and I need some advice on building/purchasing a post ban upper receiver assembly.  

1.   Should I buy a heavy standard barreled upper or match?  

2.   20" or 16"?  

3.   Floated DCM handguard or standard?

4.   Bushmaster, RRA, ???

TIA,

G45

First off, check the Brian Enos Forums on this subject. A lot of GM's hang out there. As to your questions, here goes.

1. Barrel weight. This is a personal preference kind of thing. Some people like em light and some people like em heavy. Bull barrel is too heavy. Hot ticket is the JP ultralight or midweight 18 or 20 inchers, but they are $500 for a barrel, brake and gasblock.
Your looking for a compromise between recoil control and swing weight. Best advice is to try to get your hands on a few 3gun rigs and swing em before you buy.

2.Your gonna need at least 18 inches to reliably make minor with 55's. Top guys all shoot 18's or 20's.

3.Definitely free floated but there are a lot better choices out there than the DCM handguards.Look hard at the JP handguards or the Clark carbon fiber handguards(again swing weight).

4. JP barrels IMHO are the best out there for 3gunning, but pricey. RRA turns out a really good product for the money but they don't carry a gov't profile 20". DPMS and Bushmaster does.

Your going to want a good muzzle brake. For limited, maximum size is 1"x3". Look at the JP Bennie Cooley, DPMS Miculek, Benny Hill Thundercomp or the EGW race gun comp. The DPMS brake is hard to beat for the money. Obviously you need to pin and weld it for a post ban gun.