Mi5 can definitely be competitive on carpet. I race on carpet with a very fast guy who has one. He's changed a few things on it to make it more durable and competitive though.
Just looking at my signature, you can see I'm a bit biased towards XRAY. If XRAY made a rear motor 2wd buggy, my fleet would be all XRAY.
If you choose a T4'14, the only item not included that I would consider vital for durability, is an
upper bumper brace. Even more vital if you use a hard bumper.
ECS (Dual Cardan Joint) shafts will definitely make a difference in performance, especially on tight corners. They are not as strong as the kit shafts, but can take some abuse if you build them properly. I like to use heat shrink after building to make sure they stay together even if a set screw happens to back out a little. Just trim the heat shrink down to the edge of the barrel after shrinking it. Depending on how often you hit walls, you might want to stick with the kit shafts for a bit. If you are consistently within a few tenths of the track lap record, you'll want ECS.
The kit plastic spool outdrives will wear and break, but can save your ECS in a hard crash by breaking first. I use metal spool outdrives(xray steel on 2 cars and S.P.R. Nikita aluminum on 1 car), but that does make it more likely to damage an ECS in a hard hit. It tends to take a harder hit before breaking anything, but it is more costly when something does end up breaking, because it's almost always something more expensive than a plastic outdrive would have been. A front diff would also lessen damage in a hard hit. I just prefer the feel of a spool.