I would stay where I am, continue accepting side work until it got the point that it was consistent on a week to week basis. From there I would start looking at what that income is on average and use that as a basis to start looking for space based on over head. My thoughts are a joint venture would yield the best result.
I see so many people looking to go into fabrication and modification, but my feeling is that maintenance is where you want to be starting out since it is quick in and out and somewhat consistent. You need some sort of money flow while you work on that 1 car that will take 1-3 weeks to prepare pending part availability and difficulty of the desired work.
The issue is the time wasted in a small shop setting keeping cars mobile and shuffling them around depending on parts or waiting on second party.