Carry a selection Maxon motors with gear-heads and encoders. Also motor mounts. Yes, they are pricey but if you have the budget they are worth it. But you can by directly from Maxon you say? Their product configurator is overly complex.
EDIT: I finally got through their configurator and their motors are like $1000+ O.o
If you can't get them for less then get one of their competitors. That's too much.