"balloon" tires would probably work best for sand. Think about the wheels on a garden cart. I think you can buy them without the cart
and planetary transmissions direct driving two wheels. You'd probably want it to move slowly, so lots of reduction would be nice.
You kind of need to figure out the rest of the requirements before you get too far. although it looks like this might be another one of those backwards design projects--start with the materials, then figure out a use for them. Not quite the normal way engineering works!
btw if you have water nearby, you should consider making an underwater ROV, they're also fun, and you get a whole new set of challenges to overcome. We ended up building a pretty spiffy rov last year
