These answers have all been very helpful. I have a bit of small engines experience, but I'm starting to think I'd like a brushless electric instead. I would like to drive in mud/dirt/offroad. I'm looking at the Traxxas slash pretty seriously, but still have a few questions, here's my question list:

* Can brushed vehicles be converted to brushless/ is it a difficult or expensive conversion?
* Can brushless motors be geared down for lower speeds?
* How do four wheel drive cars perform compared to two wheel drive?
* How does the scale effect the vehicle performance (1/10 1/16 etc)
* Should I get a spare battery separate from the included stock battery, and if so what should I get?

Thanks for your help, and sorry for the noobish questions..!!