I want to buy a medical office building to make a primary residence. Is this legal?

You don't want to live in it , correct? That type of change from a professional use to a residential is typically y considered a revision to a "lower use" and usually granted. The other way around, not so much.
John H Ackermann, CCIM. Commercial Real Estate Appraiser & Broker

You would have to go and consult with the city authorities at New Port Richey to see if such a use as you desire would be approved and acceptable.