Mine has 10' walls, 4-12 roof pitch with scissor truss, eave entry, high lift door.
30' long + 2 foot overhangs.

Doing it that way left the somewhat unlikely future possibility open for either two hoists or a loft/mezzanine across the side that isn't
occupied by the hoist.

I have forgotten my interior height at the peak but believe
it's around 14'6" to 15". Most vehicles (except for vans) will be taller in the "greenhouse"
area than elsewhere which the scissor truss accommodates.

If I was doing a flat ceiling I would want at least 14' in the hoist area, wherever that is chosen to be.
12 is workable but it depends on what you're working on. For cars (only), you can probably get by with 12.

I'm not sure on the insulation, a spec of 2" without an R value given sounds inadequate to me (but I am from Michigan, we have to be picky about
that stuff or we will freeze). The first couple feet of my eaves are R14 to leave ventilation space for rafter baffles, the rest is R38 + I blew about 18 bags of cellulose on top of it all a little at a time as I closed up the ceiling. The walls are dense packed cellulose. hydronic radiant floor heat.

Consider insulating the slab/vapor barrier no matter your location.

By the way, you don't need to connect water to heat the slab with hydronic radiant.
You can do a self-contained system, fill it and forget it. Mine is set up that way.
I actually run a special anti freeze/water mix in it in case the power goes out and I'm not around, don't want it to freeze.

highly recommend www.garagejournal.com for ideas and to share your build if you're so inclined.

Good luck