The fan motors use a lot of juice and are quite heavy, plus i think theyre times slightly, which means theyre designed to spin faster in one direction that the other. This is less than ideal as im sure you can appreciate. I would be tempted to go for something like 4 drill motors 9one on each wheel) for a decent pushy rambot. SLAs? as large as you can fit in the design and weight. Bigger batts mean not only longer run times but faster acceleration and faster turns. So go for ones as large as you can get.