Well I can't afford a car, so thats me out of the picture for that one. I've found the jobs involving driving to sites pay on average 2-3x more as far as IT support goes.
Not sure about 2-3 times, but yeah they definately pay more and will usually provide a company car. Mine will either buy me a new car and give me a fuel card, or let me use my own car with an allowance and a fuel card to go with that. I chose my own as i like my big V8 Mercedes, but in your case you could just let the company buy you a car.
Search around, ive never seen an onsite job which doesnt cater for a car.