using quark 125 monstor, which motor?

The higher rpms won't necessarily get you more practical speed. I think it's pretty much agreed upon (here anyway) and I have found in my own experimentation that around 30,000 rpm is where you want to be with a truck; or even a buggy for that matter. If you try to attain speed through sheer rpms, it usually means more heat. A better way to attain speed is perhaps running a higher voltage with a lower turn motor and gearing up.

On the other hand, if speed is your primary concern and you aren't worried about extended running, then a high kv motor may just be for you.

What kind of speed are you looking for anyway.

As for which Neu motor, 1512 or 1515, I discussed this at length with Mike (RC Monster). I was asking about a motor for my 11-13lb (depending upon battery configuration) Muggy. His opinion was that a 1512 would probably do the job. Something in the 1700-1900kv range. His recommendation, however, was to go with the 1700kv 1515 and use voltage and gearing to get the speed that you want. The 1515/2.5d running on 5S would turn right at 30k rpm and have monster gobs of torque for turning a big gear.