Becareful of the kit you buy. Nitro cars require more support equipment to run. Fuel, glow starters, and a starter box (in some cases) are examples of the most basic. HPI make some of the best "ready to run" kits. They have everything you need, minus the glow starter, fuel, and 'AA' batteries. They also have some of the largest aftermarket parts available.
If you are going offroad I would suggest going with a 4x4. The top end that you loose from the 4x4 is not missed, but the handling is much better. I would suggest going with a 1/10 stadium truck to get most versatility.
Stay in the 1/10 scale class to keep the cost down.
I have a nitro 1/8 on road, nitro 1/10 on road, nitro 1/8 off-road buggy, and nitro 1/8 monster truck and out them, my favorite to run is the 1/8 buggy. The buggies are pretty much the same as most stadium trucks, except the trucks tend to have a wider wheel base and larger tires.