I doo agree with what Sammus wrote. 1500 Watt is about the maximum you will need.
But i also could spin all 4 wheels on tarmac with 700-800 Watt power system:
video
With a better power system i could have outperform any nitro powered buggy (1580 Watt peak on this run):
video
The maximum peak i recorded in my 10lbs buggy was 1800 Watt. And i do see power peaks of up to 2500 Watt with the same power system in my truggy.
So i would say: 1000 Watt could suffice (depending on the track and grip condition), but to be on the safe side shoot for 1.5KW max. It's always good to have more power and dial it down (ESC or remote), then not having it.