Your Garage Heating System
Cuda, you might look around for a "guy like me" in your area, by that I mean a REAL hands on HVAC tech that knows WTF is up. It is VERY possible that just a very few components, maybe just the head elements themselves, or maybe the motor, need to be changed to get that up and running
If the wattage/ BTU rating is similar to the link you posted, it probably only has one or two heat strips.
It should not take much to get that running.
Can you confirm the BTU rate on the unit, and how much BTU do you feel you really need?
In other words, say it's got 3 heater strips in, you might get by with only running two of the three, as example.
Can you post the installation manual/ wiring diagram of the thing?
ON the other hand, if you have all your receipts, and this one contractor "did it all" it's probably time for small claims.