Here is the short version:
# 1 would be whomever you pick to build it, make sure you go and see a few of their recent buildings and talk to the owners of the building.
#2 would be to make sure they know how to pour concrete if they are doing it. On mine all they did was put in metal strips in between the 3 stalls and there are cracks running all different directions. It needs to be done right: slots cut in the concrete every so often so it cracks at the bottom of the slots. Mine also is not even close to level. The slab they poured out in the front of one bay doesn't have any fall to it so the water runs back into the shop. They came back and glued down rubber threshholds to stop that and put them on backwards so the water that ran down the overhead doors ran into the shop.
I told them I wanted 8' high doors and after it was built they had to install special low clearance doors because they didn't build the thing tall enough.
The roof leaked at the peak in one section because the roof panels were cut too short and didn't protrude far enough under the crown piece.
The only other thing I can think of that I actually did right was to have a piece of big conduit in place before they poured the concrete so I could get the underground wires into the shop for the interior wiring.
I would also bury some big metal pipe with a flat plate that had threaded holes into the floor before they poured concrete to mount a tire changer, tube bender, etc.
Good luck and make sure you have everything down in writing that you want done.
DO WHAT YOU LIKE - LIKE WHAT YOU DO
Explain to me again why enjoying life when I retire is more important than enjoying life now?