Nothing wrong with being nervous the first time you drive a much larger vehicle than you're used to. I'd be concerned if you weren't nervous. Nervous can prevent you from doing something . . . BAD.
Arrange it so that the first time you drive it, whatever you buy, it's in a safe, low stress situation. If it has to be driven any distance on purchase see if you can get a friend with experience drive it for you. Or, as in my case, I arranged with the bus company I bought it from to have one of their drivers deliver it to me to get around the insurance problem.
Fuel mileage is very dependent on what you buy but it's pretty safe to say it won't be good even for a shorty.