Updating From Oil/Steam to Hybrid Heat Pump in MultiFamily Building

Hi there, hopefully someone with more knowledge in this area can offer some guidance.
I have a 4 story, 6 apartment building with a store on the ground floor. Currently there is an inefficient oil boiler (~30 year old) and a steam system. Costs are too expensive, as you can probably imagine.
The apartments are subway-type apartments, 600 sq feet in size, with 4 windows. There are current 2 radiators at each end of the apartment on a single pipe steam system.
I would certainly prefer to offer to Tenants the ability to control their heating and preferably have them pay as much of their share as possible. Doesn’t have to be the entire cost, but at least some part of it so they can have a significant incentive to use heat rationally.
Thus, I am looking at a hybrid heat pump solution. Because of the layout of the apartments, there would be 2 heat pumps in each apartment, each being on either side.
I have already ruled out air based heat pumps because temperatures can get pretty cold on some days, here in New York City. Also because I prefer an abundance of heat rather than j ust enough of it.
So the alternative is a water sourced heat pump. I’m not entirely clear on how it works but from what I understand it requires me to supply to each heat pump a source of warm (85F) water. No problem with that, as I can install a high efficiency gas water boiler in the basement. I would replace the current steam pipes (at each corner of the building) with 2 smaller diameter water risers (same location; at each corner).
I assume that Tenants would share the partial cost with me; I would pay for the gas water heating and they would be the electrical costs associated with running the heat pumps in their apartments. This is exactly what I’m looking for.
The vertical heat pumps (Whalen) that I’m looking at seem just small enough to fit where I want them to be located.
Has anyone done something like this? What are the chances this will work, and work well at heating the apartments? Any better ideas?