When I first got my D4 I ran an
Ezrun system (5.5t) which I got off Ebay. It was pretty much perfect for the car - fits nicely under the shell, good turn of speed but not wildly uncontrollable.
It works best with Lipos, but can be used with NiMH's if thats what you have. You'll want pinions in the 18/1920 region, depends on tyres, terrain, batts etc as to which is best, so get all three!
Don't be worried about the buy over ebay and from Hong Kong bits - delivery is normally about a week, and failures on EZruns are incredibly rare.