For drive motors most people use these or a similar variant

http://ranglebox.com/product/1000rpm-gear-motor/

They give great speed and torque for beetleweight robots and are quite easy to use, for mounting them they have two holes on the front for face mounting, in my beetle theyre bolted to the sides of the chassis then the wheels are on the outside of the main chassis, but there are other ways you could do it.

Goodluck!