I use the Speed Passion range, although the orca stuff in pretty good as well.
For 4wd I use a 5.5 turn motor. However this is very quick. You could start with a 7.5 and go up once you get used to the speed, or use a 5.5 turn the speedo down and incease it once you require more speed.
Any of these products would be suitable.
http://www.telstar-racing.co.uk/prod...speed-passion/
http://www.telstar-racing.co.uk/prod...tegories/orca/