With todays buggies, it is hard to make a poor decision w/any of the major manufactures. I personally would opt for Associated due to the fact parts avail. as well as you could always buy Mike's kit for it....
As far as diff fluid goes, in my mbx-5 I am running 7F 12 C 5R and it works well for me. I don't race, but for what I do it works great running a 1512 1.5Y on 5s
good luck w/your decision!
1. MBX-6 T8 1900KV, RX8 ON 4S
2. MBX-5T 1520, MMM ON 5S
3. MBX-5 ONROAD CONVERSION 1515, MMM ON 5S
4. MRX-3 ON ROAD CONVERSION 1512, MMM ON 6S
5. TEN T 2650 T8, MMP ON 3S