It would depend on the scale of the business you're trying to build. If you want to have a full office building/staff and more than one unit, then getting both Player Run Business and Vehicle rewards would be required.

On the other hand, if you'd be content with a home office and running a one-man taxi service, then just the vehicle upgrade would be enough! There is already a character in-game who's done this (Kaito's running deliveries) and only the vehicle was required.