IMO, the BF1 is easiest to drive, bump the TC to full, slam the downforce, and even an 8 year old can drive it. (no offence to any 8 year olds who can drive the BF1 without as mentioned )
As for "learning the drivetrain ropes" so to speak, I'd go:
RAC = No one really likes driving it except for a few who like its challenge, so its rarely given a decent skin
LX = I suppose its the amount of space thats the problem, not worth skinning unless your, again, a fan of driving it
RB4 = Its skinned just as much as any other car Impreza WRC, Lancer WRC, anything WRC, look at the STCC/UKCTA, loads of RB4 skins!
MRT5 = Hard car to find a good skin design for, I think anyway.
4WD makes all 4 tyres drive, and the front 2 steer as well, so tyres get heated up quicker. Add that to the heaviness, and you wont have a long lasting car for endurance.
The RB4 and FXR would be sumo kings, 4WD and tons of grip
Demo derby was fun when S2 first came out, the servers were full of Car Parks with XFG's looking like mush, I suppose most people are bored of it, but if there was to be a dedicated demo derby car (Stripped Ford Escort anyone?), I'd happly play