The Mugen will be the better of those cars assuming it's in good shape.
The Nova .21 BF is a great motor, especially for someone just starting nitro. Alpha makes a good motor too from what I hear, but I've never tried one. I personally run a .21 BF in my buggy and I really like it, especially for the price.
If you want a faster motor, look at the Werks B5. It's a rocket for $200, though probably too much for buggy. I'd go with the B6 over the B5 but it's a little more.