(Got your e-mail ...) If the foundation is in good condition, then you've saved yourself some expense. There are some things you might be able to do yourself if you're up to it like papering and shingling the roof. If you still want a 2-car garage, by today's code, there may have to be some structural differences that may cut down your 'space'. For example, if the 'run' is too long, you may need center posts. Then again, you may not. You will need a permit if a pro is going to do it.
As for doing this type of job yourself, you can see the results of what can happen to a structure if it's not built to the right codes. Again, there are a lot of issues that, respectfully, you're not familiar with (nor I). If you know someone who is a carpenter, maybe they and a couple of your friends (or the 2 of you), can do this. My point is that the 'point' person SHOULD know what they're doing. As for how much help you/he needs, I'll leave that up to the Project Plan. You may save some $$$ buy purchasing and delivering the necessary materials too. Maybe your carpenter can act as a 'consultant'. He tells you how to frame the walls, then goes home, and the next week comes back to tell you what to do next. That's maybe 2 hours of his time but 15 hours of yours. I've done this with someone willing to do it (that's the challenge.) If you don't know a carpenter whose's able-and-willing to do the job or consult you, then if you have friends in the plumbing trade or electrical trade or whatever, maybe they can give you a recommendation or 2. Network around.
If you do this w/o the 'right' help, not only are you risking your garage but you're risking a life or 2 and any cars in the garage. For now, my best to ya and hope this helps.