Any of the associated RC18 series can be converted to be another, be it MT/T/B/R. Check the link in my signature.
Losi also makes 4x4 in 1/18th scale (LST/LST2/Raminator/Rammunition/Mini-Monster Baja), and right now they are running a
promotion that will include a free lipo and charger. Some of the losi 18th scale rtr also include a dsm radio system.
Don't weigh out the Duratrax Mini-Quake either, I have my eye on those as well, they seem to be a good option.
Either Losi or Team Associated are great choices. I have 2 Losi Mini-T, and one rc18b that I converted to an rc18t. The thing I like most about rc18 series are the availability of rpm parts (arms/towers/bumper). These are commonly broken parts, and since replacing them on my rc18 I have yet to break them. I broke my stock rc18b shock tower by driving it in the house (brushless power here) and taking off too quick and it lost control hitting the couch. Snapped like a twig.
Overall, your biggest enemy will be ground clearance. Neither of my 18th scale will go off road in any sense unless its very short grass.
Hope this helps, 18th scale can be a lot of fun and very cost efficient. Hope you find what fits your needs soon, happy bashing!