Water heater?

You can get it on amazon, dilute it with water in a pump sprayer and spray the baseboards on exterior walls, and spray around the outside walls on the house and you should be good to go.

To replace a bad water heater might be $40 for the 1st hour plus $25 for each additional hour labor plus materials.

We had the existing tenant sign a new lease prior to closing at $1050 per mo, tenant pays all utilities including water and shovels/mows. 

Little things that can add up to  be big things financially like garbage, water & common area electrical, landscaping.   

The duplex would also be close enough to the road to access city water and sewage, where as the rest of the property is not close enough to access them. 

Based on this post and your other post.You're firmly out of DIY waters here, and might require an amended 2018 return.

New flooring, interior and exterior paint, new roof and all new water heaters.

Most or all of of the water will be gone by the time it gets to the end which is why there is so much gravel all along the run.

You also want to make sure if you've included water and electricity based on one person living there, you account for the extra consumption-if the tenants pay water and electricity, this won't matter.