Chris, congrats on breaking ground! I'm just catching up on this whole thread and it's been fun to see the evolution of the project.
I've been operating out of a 1200 sq ft shop (about 400 office/bathroom, 800 shop) for the last 10 years. It's typical industrial condo construction with insanely tall ceilings and a shotgun layout (24 x 48). No problem fitting lots of Fiats in nose-to-tail but the front office really chokes things off. Your two door layout looks much easier to deal with shuffling vehicles around, yet you could still pack them in two Fiats deep which is great. Air circulation should be good too. I tuck my machine shop area behind the office to obscure the valuable stuff from prying eyes when the rollup door is open, and it keeps the long half with the rollup door clear for cars and activities. It gets damn hot in the back during summer though!
I try to keep as much stuff on wheeled carts/stands as possible to make reconfiguring the shop easy, even if for temporary work. Non-running cars on dollies or rotisseries keeps those mobile. The heavy machinery at the back never goes anywhere so no need for wheels there.
I would love storage lifts but even the smallest ones seem huge for a Fiat, where the extra width taken up would offset the gained floor space of stacking the cars. Bendpak makes some very nice scissor lift type storage lifts that don't take up a ton of space and don't require tall ceilings. I would love to make a 3/4 scale version with maybe a 3000-lb capacity for Fiat size stuff. Someday!
A sink & toilet are a must but I don't have my house next door like you.
I have seen some garage doors that roll angled with the roof so you don't kill overhead room. Might be worth looking into if that's a concern.
My shop floors are (mostly) polished concrete. Smooth and easy to roll over. Easy to sweep. Not as bright as epoxy but less slippery and no issues with tire marks, chipping, etc. Spills are easy to clean if you don't leave them too long.
I would avoid a compressor in the loft, not just for vibration but also heat. My compressor will occasionally blow the thermal breaker on hot days (100+F in the shop) at ground level with plenty of clearance around it. It's a Kobalt 80-gallon, 4.5HP piston type, vertical compressor that has (other than the occasional tripped breaker) been reliable and easily keeps up with most shop activities. It doesn't take up much floor space. It is quieter (lower frequency at least) and cycles a lot less than the smaller hotdog or pancake compressors.
The more electric power the better but one person can only use so many pieces of equipment at once
Modern inverter welders are easy on the power requirements; output amps don't equal input amps so just check the equipment literature. My compressor, TIG, and MIG welders all have plugs and take turns on the same 30-amp outlet. Not as convenient as dedicated wiring but for a shop I rent, it was low cost and flexible. If you're not welding, I guess your biggest power requirement would be a lift or compressor.