Self-steering axle, or swivel pan, surge brakes with a lockout pin as it makes it easier to back. Basket straps to tie down, or plastic coated cables to pass through the wheels. I added magnetic lights to the car for better visibility. Make sure it is wide enough to allow your car to fit. If I had to do over again I would have loved to have removable/tiltup ramps.
Craigslist for a used one if you can but good ones go fast.
When using straps if you stop the car about an inch short of the block, put on straps then tighten down they will pull car forward and never loosen.