Unfortunately, as John implied, this is one of the disadvantages of not having capital.

Your wording seems to imply you want to do something not above board. 4155.1 4.B.2.b FHA Requirement for Establishing Owner OccupancyAt least one borrower must occupy the property and sign the securityinstrument and the mortgage note in order for the property to be consideredowner-occupied.FHA security instruments require a borrower to establish bona fide occupancyin a home as the borrower’s principal residence within 60 days of signing thesecurity instrument, with continued occupancy for at least one year.

This implies that if you do not have an existing STR that you will be very challenged to get a STR permit (unlikely to happen).  

We have had a discussion with him broaching the subject of seller finance and he was very open to the idea implying that his biggest motivation for selling is to be able to get into more commercial real estate and leave student housing behind to free up some of his time and enable himself to focus on growing in other areas of his business.

This implies your limited assets go much further locally than OOS.You live in a market that has produced near top in nation return for buy and hold for this century (source Case Shiller). 

This is mostly going to be catered to horse trainers that offer lessons to the general public, but someone could also rent it out just for their horses.

If not expressly written, it was almost certainly implied, that the agreement was IF YOU bring a buyer, then you get a piece of the pie, You may not have much of a case to pursue here. 

Additionally, your new tenant may want improvements made to the office space (which are then effectively billed back via the lease rate, and this can be a source of profit if your funding cost is below your implied financing rate for the lease).

@Ivan Ng I didn't mean to imply bad intention on your part.         

I suspect this is true for Oakland and Sacramento.The post seems to imply that the property is currently a SFH.