Windows should operate properlyIf other appliances don't work well, if there are slow drains, if you have leaks, faulty outlets, etc - this is what you will keep running back to the house to fix.If you have lived there for some time, take a piece of paper and write down some of the things that really bothered you while living there.

We have a small rental property that had a tub drain leak causing extensive damage to the floor/subfloor.

This is how its done.As a rental rehabber (for my own portfolio) I just need; a decent handyman who can do simple electtrical fixtures, plumbing fixtures, fix leaks etc, an HVAC if I plan on replacing the system, a roofer if I need a roof (rarely), a painter, and a lawn guy to clean up the lawn.   

I once went to look at a purchase and the owners' ice maker line was leaking on the floor.

I was back out there in 6 months replacing them because the gaskets leaked or the handles broke.

 - bitcoin mining can be energy intenseIn addition, I want tenants to report any leaks as soon as discovered.  

This year we had a heat pump cleaning, testing refrigerant charge (didn't need any added) and a capacitor replacement, bill was $157.05, factor I consider are the age of the existing unit, if it needed a charge, potential leak, cost of refrigerant (over the years there have been many varients of the type of refrigerant). 

I've sold rentals before and almost always was asked for a copy of my Sch E.Also during the due diligence and inspection, try to talk to the tenants directly to confirm how much they pay, and also if there are known issues that haven't been repaired (roof leaks, frozen pipes in the same area every winter, etc).

There must be a cause for mold growth, and that cause is almost always moisture.Check there are no roof leaks or attic window leaks.

Tankless WH cost has dropped significantly, they tend to last longer and are less prone to leaks but the initial switch might require upgrading the gas line with potential higher installation price tag.