It currently sits in the rehab stage and I'm thinking I should I be searching for a trustworthy property management company who can handle the rehabbing process for me because of my distance from the unit.

I have not had much success as the town does not want it to be zoned as a triplex (even though there is a huge shortage of rentals, it is walkable/right next to an MBTA commuter station, and Massachusetts is now allowing home owners to renovate single family homes into having "accessory dwelling units" but apparently does not apply to duplex to triplex conversions.) 

Any suggestions or recommendations for the safest, nicest areas that fits that commute?

Trying to do this long distance is often a recipe for disaster as neighborhoods even a short distance away can hold vastly different property values. 

Nice neighborhoods, nice schools, probably decent commute for you. 

That aside, I’ve been impressed with his responsiveness and overall professionalism.For those of you with experience hiring GCs from a distance, especially for older homes in working-class neighborhoods, what questions do you recommend asking?

Quote from @Min Zhang: I’ve been in your shoes early on—managing from a distance can be tough without the right support.

I would use AirDNA (and other similar sites) data for all markets meeting your distance criteria, and then connect with STR specialist agents / property managers. 

I’m new to real estate investing and have been actively learning by reading books and listening to the BiggerPockets podcast during my work commute.

I could also keep the land and put another office condo on the front… but given that it is next to a large apartment complex and along a major roadway (with many commuters travelling to/from work), I feel like there are other businesses that would value the high visibility more.