You might be best served by having a few contractors come out to the 'garage' and give you bids. Then, when you get the estimate, ask your questions as to "Why this?" and "Why that?" and so on. If you have more specifics, then we can help.
You'll need to insulate, get plumbing installed, possibly all electrical, cable, phone, computer wire, etc.. You'll need a permit for a lot of 'things'. And if the floor is concrete, that will add to the expense. Good luck with the bids. My best to ya and hope this helps.