Taking the brushless plunge......Help

[tonkabilly]

Hello all,
I have taken the plunge and am converting my Savage to brushless. I have purchased the FLM forward conversion (on its way) and I am going to buy the MMM ESC.
The problem I have is that I can not figure out which motor to get. I will only be bashing around and do not need huge speeds. 40MPH would be just fine. Have not picked out LIPO's yet but will probably go with Zippy 5000 packs between 4s-6s.

I am leaning towards the Medusa motors. Should I go with 80mm or 70mm and what KV. I am totally new to this and have been reading for days.

Any help would be greatly appreciated.
Thanks