I would say the quandary of the request was the 15 Hp motor and what type/motor was already on the Bridgeport/ machine requirements. If you have a single machine, or need variable speed, than a VFD would be the way to go and probably the least expensive (~$150-300). Alternate option would be to build yourself an RPC or purchase one (used/new) if you do not need variable speed and have multiple machines requiring 3 phase. The VFD option is fairly straight forward, and not too difficult to install. You could use single phase 230V (50-100A) sub panel in the garage, or possibly a 30A dedicated line depending on the motor size.