If a tenant tears up a kitchen when leaving or you need a new roof, there goes 4-5 months rent. 

Lately he has hired out more and more things - roofing, electrical, plumbing, tiling.

I can tell you broadly that a lot of cash buyers avoid roof and foundation issues due to the cost and that they favor properties that require basic cosmetic repairs.

This also doesn't touch on CapEx reserves and working capital, which you'll need since the roof is 22 years old from what you have said. 

Foundation, roof, rehab...

This type of tenant rarely calls the police on a landlord, and the police have better things to do.Providing a basic, sanitary, safe roof over their head, with heat, and a place to cook for a reasonable price, is not being a slumlord.  

Roof was replaced recently without special assessment.We're trying to decide whether to sell late in the fall, rehab over the winter to sell in the spring, or continue renting after tenants move out. 

Sometimes with fha etc. the seller rolls the roof repairs into the sale price.

If you're going to need a new roof or boiler soon, it could be much higher.

Was driving around looking for properties and came a cross a house that had fire damage the roof is pretty much gone.