I'd use a car like mine. They are cheap to buy, cheap to upgrade and maintain.
You should try auto-x first and see how you like it and get a feel for the car since it is much cheaper than running at the track. If you crash at VIR and hit a guardrail, you are sent a bill for repairing the section of guardrail you messed up.

Also, plan on spending a lot of money. Road Racing is a very expensive hobby.