That's a nice size for a garage/workshop.
I can't recommend any particular supplier, but off the top of my head I would recommend these points to consider:
1) That you go with the 10' (or better yet 12') sidewalls...just because.
2) That you have a man-door next to each overhead door.
3) If you plan on having any windows, install them over 6'high; such as 1.5'tall x 4' wide, for security reasons yet still giving you natural & free light.
4) Provide for adequate ventilation. By having 1' eves, you can utilize soffit venting with a ridge vent. If you plan on eventually installing a ceiling, ensure that you provide some sort of air exchange for the enclosed space. A "whole house attic fan" is awesome.... Ventilation is an important concern regarding the escape/removal of moisture/humidity (preventing rusting inside), and secondly, to allow any exhaust or welding fumes, or paint vapors that may eventually occur inside a place to exit.
5) That the floor be raised at least 1' above the surrounding ground with gravel below the eve drip line and a gravel leader feathering any water away from the building. The last thing you need is for rainwater, snow melt, etc. to be trying to get inside your new building. Include in your planning where you will plow/place/leave snow banks - if possible, at least 10' away from the building.
6) These days, I'd say the best way to heat a work space is with radient floor heating. So, if the budget allows, when you pour your concrete floor inside the 'heated' portion of the building, do some design reading/homework & install some radient floor heating pipe. Then later on you can install a small boiler of some sort to heat the workspace via hot water heat circulating within the floor. Radient heat is quiet, dust-free, spark-free, no fans and no ductwork.
Here is a great site for more ideas & advice: