The HPI EVO's have metal diff gears. The 18SS & EVO+ have the higher gear ratio better suited for topspeed.
The 18tz is spose to be a brutal little motor...especially modded. Ive also heard good things about the TRX 3.3 with the parts from RB Mods on it.
Since you just parking lot bash, you dont have to stick with the .12-.18's...if you dont want to. They accept .21's just fine...
I have a RS4 EVO+ with a modded Hyper 21 8port in it. I tested it once with the 47/43 spurs with 19/23 pinions just breaking in the motor and it was still a beast. Im waiting on 41/37 spurs to show up, and hoping to find some GPR 44mm rear foams for it soon.
The same motor in my firestorm has been 63mph turning 39000, so im hopin for at least 75-77 with the car, goal is 80..for now.