If it was me building this diorama, I'd concentrate fully on the General Lee first. Once completed, with or without paint, you can use it to gauge much more accurately how wide you want the road, or how long the ramp is going to be etc etc. Then you can draw a basic 1:1 birds-eye-view plan on paper, and work out your measurements, rather than hoping they fit the vehicle after.
For a base, I've been experimenting with a range of materials. From my somewhat limited experience, I've tried cardboard, MDF, and polystyrene. All have worked, but MDF looks by far the best on the final product.
As an alternative method from Richy's, you can also use Modroc to form your base. On my dioramas, I tape down balls of newspaper into the shape of my terrian, then lay Modroc (or Terraforma, I think it's also called. Basically, plaster sheets) over. This is very easy, very cheap, and gives a realistic looking 'bumpy' effect. From there, just paint it a basic colour, and do whatever you want to create the final land effect.
Those are a few basics that may get you started. There's countless ways to make dioramas, so don't be afraid to experiment. Good luck, and keep us updated