The first gen Trooper is a classic baja proven vehicle. Not sure how it would stand against modern vehicles though.
Get a second gen Rodeo and if it doesn't have the 3.5, build the 3.2 into a 3.5 (since the 3.5 is really just a modified version of the 3.2). Alpine makes a supercharger for that engine, if you can still find a kit, but it's a little pricey at over $3k. Not sure what the rules of baja racing are, though, so I don't know how much you can modify the engine.
On any of these, you'll want to beef up the suspension. I'd look into what Joe Darlington, Ditchdoc, and Bansil have done w/ their Isuzu race rigs for the ECORS races.