How long will they be able to pay the heat in a bad winter? 

Does a hardwired smoke/heat/fire alarm system count towards that?

That's the general idea yeah, but if it's owner occupied and he's going to lose the house anyway, why wouldn't he at least try to work something out with you?

The age of the roof doesn't really matter, as many things play into how a roof performs, the type of roof, heat, rain, wind, etc.

Do you pay water, heat, electric, gas, etc? 

Just for a better picture of the project: it is to install new central air (heat 80% for the first floor and 92% for the second floor & AC 2 and 2.5tons) in the both units of a 2-family house, that guy proposed using Rheem units and keeps telling me that the brands like Collman (Coleman?)

Ask them about problems , bugs , leaks , heat , and AC .  

Might skip that one and just feel better about a purchase.Or, get it inspected and know what you're looking at.Search; knob and tube, plaster and lath, replacing old moldings, That should point you in the directions of concerns, there are HVAC issues, boilers old gravity heat systems, window issues as well.

You might lose a furnace motor because there is no air going through or even crack the heat exchange box and require a new furnace. 

You can try a dehumidifier and put some heat on and fans to it, you can see, but it may be had.