I'd recommend a two stage, 7.5 hp, 80 gallon with atleast 15 CFM at 90. Bigger would be better if you plan to blast regularly, but that should get you by. A compressor running all the time is not only bad for the motor and compressor, but it also creates heat which will in turn create condensation, which will be bad, very bad with a blaster set up. With blaster, you need volume not pressure, I never use over 100 PSI blasting on frames and thicker metal and usually around 85-90 on sheet metal. Of course what size nozzle, air line, filters, water traps, whats being blasted, and media used plays a big part in how much pressure you need too. Maybe you could check the other shops in your area and see if they will share information on their set up.
best of luck with it,