Separating Heat and Hot water utilities for a 2900 Sq Ft 4-plex

[Sravan Tatikonda]

I'm currently house hacking in the North East. The property that I'm looking at was a single-family one and it was converted to 4-plex: 1 Bed/1bath(400 SqFt), 2Bed/1bath(500 SqFt), 3Bed/1bath(1300 SqFt), 2Bed/1bath(700 SqFt). Currently, Heat and HW are paid by the landlord costs around 4500$ run with an OIL(15yrs old) based furnace and a 60 Gallon hot water tank, all rooms are equipped with baseboards. All units have their own thermostat. Electricity is already separated.

Please suggest to me some affordable ways of separating utilities for a better cash flow. 

As the furnace is old, I'm looking at it to replace in a year or two with a natural gas furnace, I may be able to get it done with separation work if it's cheaper.