for off roading i would get a nitro buggy/truggy, stadium truck, or a monster truck
hpi make good trucks and they are strong and spares are readily available for them
www.hpieurope.com
hotbodies also by hpi are good cars too but i dont know what they are like for spares etc
http://hbeurope.com/
take a look at what these nitro trucks can take
http://www.beatyourtruck.com/beatings.html