Before you do ANYTHING, decide if you are going to insulate and heat it, and COAT THE FLOOR. It will only add time to getting back to working on cars if you have to tear down a bunch of shelving/stuff to do that later.. Ask me how I know..
Or..
Buy a house where the old garage was converted into a bonus room/office, and convert it back to shop space. Central heat and air (sep from the main house HVAC), nice view of my yard, trimmed, painted, even a nice ceiling fan..
Even when I did this, I made sure I epoxy coated the floor before putting anything in there. So much easier to keep the floors clean. Simple green and a mop. Floor looks great, spot shoot it with concentrated Simple Green to get grease and oil off the floor.
Sadly, the shop is full of shit right now, benches, TIG mIG, plasma, drill press, band saw, lathe, shelves, old Triumph (bike), couple of motors... Mill is still sitting in the garage.
Other things:
Run enough power out to the shop. Even lots of flourescent lights will eat 15 amps quickly. Run at least 50A 220V if your meter can handle it. I ran 100 out to mine on top of the existing lights/outlets.
Doing this stuff before starting a car project is totally worth it.