as said great idea on paper however in pratice there are so many issues, first off insurance, theft, broken equipment, time overlaps etc etc the list goes on.
if i were you i would do one of two things:
firstly, only rent it out to people in the trade, like mobile mechanics or car dealers, they should have adequate insurance for themselves plus know what there doing and give you a realistic time frame, as good as us diyers are were all optimistic when timing up jobs
secondly, you rightly said that you'd have some one supervising, well why not employ a mechanic instead with the understand that you pay him x amount to 'supervise' and pay him so much to carry out proper mechanic jobs thus again opening your businerss horizons. just make sure you employ him on a self employed basis making him a contractor
just my thoughts on what id do really