Several important bits of advice have already been offered regarding the transaction but, alas, there are more things to consider.
Converting a garage to living space requires significant attention to the building envelope. If the modifications were done properly then a true thermal break was constructed so that the original concrete garage floors, walls and ceilings do not degrade thermal performance. All of the walls and the ceiling of a garage would normally not include insulation and may not include thermally insulated windows. Depending on whether there is living space above the garage, insulation above the ceiling is required and a vapor barrier is needed throughout. A simple inspection now, after the fact, will not be able to answer all of these important issues. Hopefully there are architectural drawings, construction photographs and other means to conduct a full investigation. Otherwise I would advise against making an offer on the house until you have been able to inspect the inside of all exterior walls, a portion of the floor structure and the ceiling plenum above. I'd be very interested in the HVAC system used for the space and whether it is serviced from an existing system or an auxiliary system.
FWIW, I have been a licensed architect in NJ and I'm currently a licensed realtor in Morris County.
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