I couldn't quite understand the question, therefore didn't answer.

Does your friend wish to rent the place for himself or is he the owner?

If he wishes to rent for himself then the rent should be part of his business case - ie: he should have an idea what the market is willing to pay to get their car fixed (and how big the market is) then subtract all the business costs & the final figure will be his hourly rate that he has to live on. No point doing a business case based on $90/hr & 50hrs a week work if the surrounding area is full of tightwad pensioners & the whole town has 10 people in it. No point having the hassles of working for yourself & ending up with $700 at the end of the week if the local garage/dealership pays $800 without the hassles & responsibilities of running a business.

When I was mechanicing, our hourly rate was 20% less than the local Toyota dealer & we had a core group of about 300 repeat customers on the books for 2 mechs & an apprentice plus we had a heap of "blow-ins". Cars needed a lot more servicing (oil change every 5000km) back then, so I assume you'd need more customers these days.

If you own the property, then rent will be what the market can take. Why limit the place to be a mechanical workshop?

Also, rent will depend on the terms of lease. Is it a short lease or something more complex like 3yrs + options of2+2+ with rent reviews every 1 yr/2yrs/etc?