Hi Stef,
The devil is always in the details, so check carefully. IF you have to deal with codes and zoning, there may be significant costs and neighbor issues if you attempt to convert a garage ( the property is next to a golf
course). Plumbing is certainly a concern. It looks like there may need to be an electric upgrade. Those costs may be better spent on a basic new build. A shell would include whatever you define it as. I am thinking floor, exterior walls, doors, window and roof.
I am familiar with the Avon area of NY. Snow melting into a garage with a slab is no big deal. It can become a concern with a house on a slab … especially if the house was originally designed to be a garage.