I'd say 28' as a depth minimum; I have cars that are over 20' in length and to be able to walk behind and in front of them with a tool box up front,(drawers open of course), that would be a minimum number. Consider putting a small ventilated room on the outside for your compressor because they are ungodly noisy. You can't have enough electrical or air hookups! I have one air hookup and 100' of hose that is always tangled up. There are places I have to use extension cords where I could have had outlets. A drive on lift is nicest but the "pit" will be much cheaper. You will need to use concrete blocks and cement I would think. Put a drain it before pouring concrete. If building on a slab, be sure to leave the concrete elevated a couple of inches to keep water out. My dad made this mistake and it's flooded every time it rains.