Any opportunites to add value like storage space you could charge for, pass the heat or sewer costs on to the tennants, are rents below market? 

Self storage...he just had an offering a few weeks ago.

I have the opportunity to purchase a 4-plex with a 1500 sqft storage building that was abandoned a few years ago. 

Water could be heard moving through the heating pipes as well, which is symptomatic of air in the system and a possible failure of devices on the boiler meant to purge air, 2.

Storage?

Perhaps it’s big enough to build a small self storage building on ?

Also keep an eye out for evidence of any lease violation.... unauthorized occupants, unauthorized animals, tampered smoke alarms, damages, etc.In addition to checking all of the smoke alarm devices, plumbing fixtures, electrical fixtures, appliances, and operation of doors and windows, ask the tenant if there's anything that's not working or needs attention.

I already know that a clean nice lit storage area would be a great upgrade, and we plan on doing that to the basement, after we paint over the graffiti/etc  https://www.realtor.com/realestateandhomes-detail/...I appreciate it! 

A roll-away center island might help with storage and give additional working counter space.

Also, I don't know the formation of your driveway and garages, but I have maximized my parking by actually blocking to garages with three cars parked perpendicularly and then renting the garages for storage.