Diorama Base

I will be trying styrofaom on my next dio so I wont be able to tell you. But the kind of plaster would be regular old plaster, I use Plaster of paris. But if your dio is not going to be on a road I would recommend Celluclay.
Check out custom dioramics for stone buildings. I know they make a Normandy Farmhouse. You could also build one out of sheet styrene from plastruct. they make stone styled sheets.

For dirt on tires I use a really thinned down Tamiya Buff. Looks great for light dust. You could also try Tamiya red brown (i havent though.)

Here are a few pics of the buff.

thanks, that helps a lot, and my diorama is going to be part road

Here's a few suggestions . I put a layer of polyfilla (it's a plaster wall filler abit like spackle ) on my bases because it doesn't dry too quickly . Move it around with your fingers or a brush. If its a dirt track then smooth out a bit with the brush but if its in a town then you have cobbles and there's a number of different ways for that. When its all dry, then you can sprinkle lots of dirt over it , I use dry sand and small stones found outside. When you have it to your liking then make up some white PVA wood glue with water & a small squirt of washing up liquid (it helps break the water tension) and drip it over the ground work until it's soaked thoroughly . Then leave it 24 hours to dry rock hard and then paint it how you wish. For building rubble ,get hold of some house bricks if you can , and smash them into tiny bits with a rock hammer . Use a sieve and seperate into small,medium & large bits . Then spread them around the building and then glue the same as ground work. The glue dries invisible and sets the brick work rock hard too. This can be used on any type of brick or stone work.
Hope this will help you and look forward to seeing the finished diorama .

I use a regualr, stained wooden plaque, with Celluclay applied to it. It's a type a papermache. that's for a flat base. otherwise, i'll use cut foam, with celluclay applied over that, but again, on a wooden plaque