The plan is weak. Generally, I wouldn't invest in a business that I don't know, myself. That would be a non-starter right there.
If I did know the business, I would want to know the costs associated with the business - taxes, insurance, government fees, energy and water bills. How much does it cost per month if the place sits idle? I would like to know the historical income of the shop for the past few years. Do customers come because they like this particular mechanic? What if you can't get one who is as popular? For that matter, what if you can't get a mechanic that is willing to guarantee you $4k or whatever per month? What is plan B if you do sign someone on, and the whole shop only brings in $3k per month the first few months? Or even $5k? The fool who signed on cannot be expected to stick around.
It also seems that the present owner wanted to rest they could hire his own staff, then sit back with a guaranteed $4k income per month, in a business he understands. It could be that all the good mechanics want guaranteed pay for their hours, not subject to the whims of the business.