Still waiting on answers to the questions I asked.....
Ultimately, when it comes to selecting a compressor to be used in a professional shop environment (especially if we're talking about a fixed location that expects eventual growth) it's best to get the most capacity you can afford and set up your air system properly (quality plumbing, regulators, filters, etc.) the first time around. Looking at it from a "how cheap/small can I get away with" standpoint is only going to cause you to keep replacing equipment -- and you will likely find a good air compressor can be the lifeblood of a shop doing any sort of automotive work. You never know when you might discover it's handy to have air ratchets, impact guns, die grinders, mini-polishers, etc. at your disposal and all of those things require lots and lots of air, especially when/if using more than one at a time.