The Mayen M8 makes an great profit even at Three Stars. It is also a fairly easy car to rebuild.
Purchase Price: $38,814
Repair Cost: $4,756 (+/-)
Sale Price: $78,169
Profit: $34,599 (+/-)
As you can see, the Mayen M8 makes for a great car to rebuild in Car Mechanic Simulator 2015.
Rebuilding vehicles for profit in Car Mechanic Simulator 2015 is by far the most profitable activity in the game, assuming that you're picking the right vehicles. Of course a solid supply of spare parts helps too. Here are a few links to additional information that can help you get started in the Car Rebuilding side of the game. This will help you improve your overall game experience:
Rebuilding Guide with Cost vs Profit for all Vehicles in Car Mechanic Simulator 2015
The Auctions Center - The place you buy vehicles
Complete Series of Vehicle Rebuilds for Car Mechanic Simulator 2015
Tunerz Paradize - The place to get special parts to trick out your rides