Because your walls are sealed you will need to use an open cell foam. 

I've used Odor-Xit with some success after a deep clean, and sealing with BIN primer, but this was on the subfloor, which I then installed new flooring over. 

If it’s just a shell and you’re going to build it out, I can’t imagine getting a permit anywhere without engineer/architect signed and sealed drawings (blueprints) that detail electrical, plumbing, walls, load bearing calculations, etc.

That sealed the deal for me. 

The pro stuff really works.Also important to seal the building.      

Matter of fact, we won't let a tenant use a fireplace in a rental--we seal up the flue. 

I am hoping to seal this up soon so I can start investing! 

You can seal up all openings in the apartments, and all openings into the building in the basement.

Then you can blame the lease when something unexpected happens.The lease/rents will be based on a portion of common area expenses, like the property insurance and taxes, the occasional re-seal the parking lot, seasonal sidewalk clean-up and roof maintenance; plus the upkeep of the tenant's space - spell out who will maintain the furnace and who will pay the plumber when the toilet backs up?

Identify where the hatch is or was, make sure it's sealed and nothing is getting in, and you'll be fine.