I've been looking at this as well, the hobby shop on base took a serious hit in the recent budget cuts and they dont have the space for the business they get now. There's one storage place in town thats open 24 hours & allows you to work in the units, but it's always booked up. Since there are always alot of car nuts around military bases there's a business potential there for sure. It really depends on the area you live in tho, might be profitable, might just scrape by. Still a good retirement plan really.

The legal thing is a big deal, as is insurance. I'd write in a liability clause that each person is required to have proof of renters insurance for long term usage, should save some on your end and will cover them if there were a fire or something like that. You also have to check with the city/county planning office & make sure that type of business is allowed on the property. It's a crock, but can cause a real problem.

The BIG thing you'll have to worry about, HAZMAT. You as the owner will be ultimately responsible for every drop of HAZMAT that is on your property. Storage, disposal, spill prevention/containment, all has to be covered. This stuff is no joke, & it gets expensive.
If some geek leaves an oil filter sitting out & an inspector decides to pop in, it'll cost you approx $20K right then & there. & no you cant fight it, it's not transferrable to the person who did it (altho they may get a fine as well). If there are contaminated rags laying about, you could possibly get a fine for EACH item, not just one fine for all of them.
Yet another clause to put in the long term storage/usage rules, they mess up, you confiscate thier vehicle to sell & pay for the fines. The EPA isn't playing games anymore & the only way people will take it seriously is if there is a chance they will lose thier car.

I'd look into all that & talk to a lawyer about what it would cost to write up agreements for daily usage, long term, & HAZMAT. You'll want them written up properly because no matter how many times some people tell you they understand the rules, they will take you to court & say they didn't.