For a scale rig I would say torue is the one to look for, go after the 6v ratings.
For modern chassis like SCX and TRX-4 etc I would not recomend less than 15kg, same with a heavy ish chassis.
For a CC01 a 6.5kg + servo would work fine.
And like Heartless said it depends on the chassis and useage.
R.I.P Ole (8th Oct 2012)