I love small business discussions. It's my passion. But I won't get deep into this one because I've never run an auto shop, and there is plenty of good advice.
There are, however, a few things common to other businesses...
First note, one of the most experienced voices in this thread (Capt) is no longer in the auto industry. As a potential owner, that's an important note. In spite of vast knowledge, I'm hearing "Nursing is better than being a tech". That's a warning sign for an owner. Techs are REALLY hard to keep.
Second, be careful of the love affair with the fantasy of the auto business. There are many businesses that look good from the outside. To an auto enthusiast, auto repair is glitzy. But behind the scenes there is an awful lot of pure grunt work and toil in making a business like this successful. My weakness was a coffee shop. Total glitz on the outside. The reality for me is it was the most fun I ever had loosing a buttload of money. In 2 years I never drew a paycheck at all, and eventually closed shop with $40,000 in debt. But I had a lot of fun doing it.
Flat rate... interesting discussion. I work in an industry (construction) where flat rate is the gold standard, and works exceptionally well for the techs. I'd NEVER work any other way. I know basic laborers who clear $1000 per week every week because of it. It's a reward for getting good at what you do. But it sounds like there are some big caveats for that in auto.
I'm concerned with the businesses you are describing. You said they are all closed. In my book, that means you are NOT buying a business. You are buying an old building with a bunch of used tools in it. Without the customer base or the business systems in place, you have a really long haul in front of you which will be slow and expensive.
Subcontracting: Again, my industry does this really well. Sounds like it works in auto too. It's a MUCH better business model than trying to keep things in-house. The best model to me seems to BE the subcontractor. When all your sales are behind the scenes to other businesses, you become the specialist, and their revenue is dependent on you. When you sell to the public, you are an expense and an annoyance to your customers. Always a fight. When you sell to a business, you become an important part of how THEY make a living- they are excited about paying you (because they get to add their markup). My concrete finishing crew is a 6 man crew that generates over $20,000 in revenue per day. Every day. They are exceptionally good at what they do, and we (and other contractors) keep them working as much as they want. The owner of their company makes plenty, and he doesn't have to sell at all, and never has whiney customers.
Just some general thoughts. I'll go back to being a lurker in this thread, but I wish you the best of luck!!