The furnace is in a hallway closet upstairs and my property manager said usually the vents closest to the furnace will get the most air.I just acquired another house also built in the early 80's and same thing, upstairs gets all the heat while downstairs is a bit cooler.  

Specifically, Water, heat, electric, garbage or any other bill of that nature? 

Here are the elements that go into my situation:1) Currently in mid-range salary job (60k).

Tax - 2.6k (6%)Landlord Gas Water Heating - 1.3k (2.9%)Landlord Gas+Oil Heating - 2.8k (6.5%)Landlord Other Utilities - 5.5k (12.6%)Insurance - 1.1k (2.6%)NOI - 11.8kLending costs - 8.6kAs is, cash on cash would be 5.5%Is (eventually) getting 10% cash-on-cash realistic here?  

I would check with a local HVAC (Hydronic) wholesaler or installer to see if you can still obtain them {be prepared for them to cost almost the same {probably more when you factor in labour} as replacing the entire unit.As for using PEX, I'm not certain if the heat-rated PEX (used for in-floor hydronic heating which runs around 140F) can be used for a boiler and radiators (which typically run around 180F).  

A hot water heater isn't a big deal, but an heat pump is, when you hadn't planned on it! 

I also pay for heated water for all 4 units and that gas bill is around $50/m.

What I am seeing is a lot more rehabs that are just carpet and paint and flippers leaving in old windows, heating systems which are on their last life. 

Heating bill and fuel type?

Lights on, tv on, heat or a.c. on.The other half of the time I get a call from someone who just came into town or I give them a drug screen in the morning to get into the refinery.