Some interesting ideas here but a dose of reality needs to be injected.
First, can you pay cash for it. Banks will not usually do a mortgage for such a small amount. Do you have a good relationship with yours and will they help you figure out a cheap way to finance it?
Roof: most banks will not make a loan on a building with a leaky roof unless the roof is repaired. And they usually want a contractor to do it before the sale.
What is the ceiling height? For a studio, an 8ft ceiling is not good enough. I wouldn't even consider it with less than 12, personally.
A lot of repairs and upgrading can be done by yourself easily enough. If (and that's a big one) you are good with this kind of stuff. You don't want to bring in customers into an unsafe building. Electricity needs to be in very good shape. If you can't do the work yourself, it will cost way more. Also, if you can't do the work yourself you are probably not able to judge of the state of things and may find quite a bit more that needs fixing after you move in...
But there are possibilities. I bought a complex of buildings not too long ago. One of them had a flat leaky roof. We tore it off and put a new metal one in place while raising it and pitching it at the same time. It was done in less than a week and cost less than $4,000. Then again, I have restored a few houses so that construction work is easy to me. Especially with the help of a few like minded friends.
The idea of laying a new roof on top of the old one is usually not a good one. Roofers suggest it because it is easier for them and cheaper for the owner so it is attractive but not the best idea. Flat roofs are the pits to maintain so if you can pitch it as you re-do it, you would be cool for much longer.
One last thought, You don't need such a large space for a small studio. Cut the space in half and rent it out. With the right deal, it might pay for the building. That's what we did with the complex I bought. There was one store in there when we bought and we brought in a second one. They pay for the mortgage. Another thing we did in there is set up a rental photo studio. The one for which we raised the roof. It is rented regularly enough to have paid for the repair already and on its way to pay for the gear...