When I built my shop, we laid a single run of concrete bricks around the perimeter of the fresh slab and then built a standard 8" 2x4 studded wall on top of that. I used wood trusses for the ceiling and roof, walls and ceiling fully insulated, and sealed with painted OSB. This combination has made a decent workshop, but the next time I build one it will be significantly taller as the @ 8'4" ceiling height limits what you can do inside. Like others, I would also build bigger (the current one is 26x36). I learned long ago to make extra wire pulls thru the "attic" before sealing, this allows for future circuit expansion. I would also suggest at least plumbing in a drain and a water line, you don't have to hook then up, just have them run through the slab/footing prior to pouring, this enables you to add water at a later date if your plans change. You might also think about running phone and/or ethernet lines in with your electric supply (assuming that like me, you live in an area where wireless service is not available), also for future expansion/different needs at later dates.