Well I have two e-maxx's one is mostly stock except rpm stuff and the other is an extended aluminum chassis with cvds etc. I don't know about idlers or diffs because I just got it used from someone. Oddly though, it seems to weigh less than the mostly stock one. I would like to go as fast as possible but like I said, I don't want to spend more than 300 right now you know 310 320 would be fine but not much more. So it sounds like the 9920 would be a good contoller for now and I could upgrade motors later. 80 bucks a motor doesn't seem too bad to me.