Does anyone have a good estimate on the monthly expense cost for running a radiator on a small multi-family unit in Ohio? 

As they are box-shaped we think they are some sort of radiator that partially burned the floors.

I had cancer a while back (treatable) but I had to get radiation which meant laying very still on a table and having them close a big door; the radiation techs were very understanding and would talk me through it and somehow I managed and after six weeks of daily treatment I barely had any panic. 

Not cheap when you have radiators to disconnect and run ducts.

I pulled back the carpets to see if there was a return pipe (as if there was an old radiator line) but there was nothing.

If the latter, I would also plan to separate the heat as common forced air is a conduit for pathogens, allergens, noise, cooking aromas and fire between units - tenant relations are usually better when those things are not shared.If the system is hydronic (baseboard or radiator), then the key bit of information will be whether each unit is plumbed as a separate zone from the others.  

Someone allowed the oil to run out and the radiator lines froze and split in a couple of places - but miraculously little water damage.When I ran my numbers I allowed for a purchase of up to $30K with renovations of 80K for an all-in of $110K.  

I got a new bathroom framed in, electrical is all set, I now need to rough in the plumbing, move some radiators around...etc..to be able to continue.

BUT if you are keeping the units for rentals, and have a hot water BOILER, by all means keep the radiators/baseboards that go with it, install a combi boiler (Navien or similar) that combines hot water for heating with hot water for showering.

sorry about that the furnace is electric from 1991 and the radiators are hot water baseboard