We installed all new HVAC (heat pumps for all 6 units), windows, appliances, kitchens, floors, doors, fixtures, and water heaters.

It’s a fluid market.

If there wasn't enough heat the weld would leak water. 

It will require new split-system HVAC for each unit, which will provide forced air heating and cooling.

, water pressure, age of heating/cooling systems. 

Age of the updates (when was the roof, plumbing, heating, and electrical updated)2. is there knob & tube or Aluminum wiring in the building (active or inactive)3. is there an underground tank (oil)4.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

There is no central heating as an option.I am loathe to rip out the existing unit to replace with a heat pump, are there any other options I can provide the tenant? 

I suggest learn a little about the building trades like carpentry. plumbing, electrical, heating a/c and even painting.

Separate Utilities.Price Per Unit $45,833Gross Multiplier 5.05Cap Rate 13.42%Number of Units: 6Construction: BrickIncomeGross Income Rent $54,420Expenses Taxes 2015 $7,531American Water 1,332Elec. 0Maintenance 5% + Vacancy 5% 5,442Gas (heat & hot water or Oil- apts)0Insurance 3,210Est.