I can answer a few of these as my hot tub is in my garage.
1. I think all you really need is the electrical inspection.
2. If you live in a cold climate, make sure the steam can escape. I open 2 windows
and it makes a big difference. That should be all you need for ventilation. Again, in cold climates your tools will rust in the winter with the steam.
I cover everything up in plastic bags. I do have a ceiling fan a foot from the edge of the tub....It's nice in the summer, but you could get a floor-standing fan and
it would be cheaper. As for plumbing, I guess just make sure you have a long enough hose to fill it.
3. I know I sound like a broken record, but....if you live in a cold climate, get a forced-air propane heater. It REALLY cuts down on the steam, and it warms up the garage at the same time. I got one off Craigslist for $60
4. I'm not sure. I haven't done anything and I've never had a problem.