i have experience with all of the quads listed above. if i were to do it i think id get the grizzly and talk to kenny at kds and get the head, valves, cam, piston, tb+3 , and HR +5. id run lte dual exhaust, 29.5" outlaws, epi clutch kit, dynatek ign and a riser. read april edition of dirtwheels, they have an awesome grizzly build in there.
a/c mudpro- too slow, but hey it is a built mud machine, reliable.
a/c thundercat- id love to see one with a 6" gorilla lift kit w/axles. no problems except they like to blow front diffs and axles. aftermarket fixes available
bf750- cheap and awesome as long as you keep it washed, keep with regular oil changes, and wash under the seat and keep the seat off while it dries. watch mountains and mud 3, 90% of the quads on there are brute forces, poor v-twin design my ass
renegade- powerful, expensive, cant brake or steer worth shit, and have weak frames, and LOTS of warranty work for the first 6 months.