Hey Paul, going up, with a gambrel roof would certainly give you a lot of storage space. I would check with local codes for foundation requirements, if adding a loft, you of course will need beams and joist for the loft floor, and a perimeter foundation to code to handle the load.
I built a 24X36 garage 8 years ago, 3 overheads on gable end wall, 2X6 construction R 20 walls R40 in the attic. I would think you do not require insulation, but when it is real hot here, that insulated garage stays nice and cool.
If you cannot build on the ground the size you want, then go up, just get a spec for building codes, if you plan to use the existing cement pad. Up here any garage over 880 sq feet needs an engineered foundation/floor. Under this you still require a re-bar, cement type, and a thickened edge on the perimeter. So do your homework first.....good luck, just clean up the truck re-wire, etc and get her going...