Are there signs of water intrusion on the inside of the structure? 

Certainly replacing a furnace and/or a water heater would require a permit as well as running a new or replacing a duct run.

For example, if a family is displaced due to extensive water damage that will take a few months to repair, the insurance company will then look to find temporary housing for their customer/displaced family.

We are leaning towards testing the waters for putting it on the market even though originally it was going to be a long term buy and hold.

Water, air, sewage?

You may be dead in the water. 

More importantly, I am fellow taxpaying citizen.Having said that, we live in a world of gray waters... a reasonable tax position is all I demand of my clients and my team.

I own a newbuild (2019) SFH rental, when build it included solar (without the batteries) and solar-hot-water-heater.

I usually fly fish Cosby Creek, and have seen 2 other fishermen on that water in 19 years.  

Renovated all apartments, converted M2M tenants to annual leases, instituted strict rental criteria, implemented online payment portal, invested $75,000 into leasehold improvements (HVAC, drainage, exterior paint, concrete.)