1515/2.5D should be fine..I have it on my 13lbs EMAXX and easily gets to 40+MPH (for bashing i think its fast enough and for very rough surface i think no need to run a that speed...will result to either broken drive shafts or burnt ESC "maybe")
BTW, you should have bought the MMM from Mike he has them on stock. I believe a couple of preorders were canceled so Mike have some spares....
RCM MMM on stock