4s Rustler, which motor should I get?

Frankly, if I were doing a Rustler, I think I'd have to give the MM 4600 system a shot. Of course, the Lehner should be better, but I don't know that it would be worth the extra expense. The MM systems, when run in their intended environment perform very well. In fact, that Rustler will be a beast with a MM 4600 system on 4S lipo.

If you just want to go with the Lehner, and 4S, I'd look at something around the 4000kv range, maybe a bit lower.