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. 

Most calls i get is probably heating and plumbing.  

In Ontario (Ottawa) people with student rentals, do you include all utilities (heat / hydro / water) in the cost for students, or do the students pay the utilities on their own (split the costs between them)?

While most Net Zero buildings are energy efficient, perhaps even highly so in comparison to minimum building code requirements, there is nothing requiring them to anywhere new the efficiency level of a Passivhaus   .... this is why you will see odd things like warehouses with large rooftop solar installations being able to claim Net Zero.On the other hand, Passivhaus sets three metrics which a building must meet to be certified as Passivhaus compliant:  The building must have a heating and cooling demand of not more than 15 kWh/m2 (4,755 BTU/sq ft; 5.017 MJ/sq ft) per year  and a peak heat load of 10 W/m2.Total primary energy consumption (energy for heating, hot water and electricity) must not be more than 120 kWh/m2 (38,040 BTU/sq ft; 40.13 MJ/sq ft) per yearThe building must not leak more air than 0.6 times the house volume per hour (n50 ≤ 0.6 / hour) at 50 Pa (0.0073 psi) as tested by a blower door.Ironically, the research and pilot projects which fuelled what eventually became the Passivhaus standard was carried out in Saskatchewan as a National Research Council (NRC) project in the 1970s (the house is still in use today) and a similar project (Minnesota?)

Here is a part of the letter, I sent to the seller:The home inspection was completed on September 29th (please see attached) and the following issues were pointed out: 1.The wooden floor frame in the middle unit is deteriorating due to either prior water leak in the bathroom or due to moisture infiltration through the walls and foundation.2.There is evidence of significant settlement under the middle unit bathroom and north closet.3.The settlement of the floor and walls has also resulted in settlement of the roof frame above this area, ceiling shows cracks.4.There is an obvious depression on the roof – water is ponding on the roof above the middle unit, which adds excessive weight on the roof and potential for leaks.5.Asbestos containing material has been found.6.The current bedroom heat installations in the north and south units pose a safety hazard from carbon monoxide, fire and oxygen depletion.7.An electrician performing upgrades did not use ‘arc fault breakers’ in the north and south unit breaker panels, which is required by the national electrical code.8.Upgrades to the electrical system, promised by a real estate agent, have not been completed yet.