The BEC is the battery eliminator that's generally built into the speed control.

I had a Mamba Monster, and the only little thing I really disliked about it is the cogging if you're trying to start the car moving slowly and smoothly. If you just hammer the throttle, it worked fine. Also, if you are rolling backward, the ESC would hesitate before applying throttle. Most guys I know that use the MMM aren't bothered by those annoyances, but I was, so I went with an RX8, and I've been very happy with it. I think either one is a fine choice, but I do prefer the smoothness of the sensored start up on the RX8; however I prefer the wiring connections on the MMM.

For a servo, I think it really depends on your budget. My $70 Savox 1258TG was the best for the money. Not quite 200 in*oz of torque, but I never had any problems with it, even with a few nasty crashes. Savox is offering the $50 rebate on brushless servos, so you can get a $150 SB-2271SG Savox for $100. Too bad it doesn't have Ti gears to make it a little more lightweight, though.

I just measured the tray in my RC8Be. It's 144mm x 54mm without the padding and 139mm x 49mm with the padding.