Did any water intrusion happen in those 15 years that might have compromised something like an electrical panel?  

The water has been disconnected for about 15 years.

Hi, We recently had water backup come up from our floor drains in the basement.

Check exterior grade, you don't want water problemsHope that helps....

Someday we’d want to add tankless water heater and Quiet Cool.It’s a highly competitive area, with low supply and had the highest growth in rent increases.

BBQ while looking at the water, fish from our backyard etc. 

You are correct that there is no one watering hole that "off market" deals congregate at. 

So the tenant in my rental was complaining that the refrigerator panel where the water and ice come out would pop off occasionally, but mentioned that it wasn't a big deal because she doesn't use the water or ice.

They already have two AC units but would we need two water heaters?

They forget long-term maintenance like hot water heaters, plumbing and roofs.