You raise $5K/door (or more) upfront to address deferred maintenance and value-add improvements.

In the warranty deed that was executed the seller swore to defend my right and title to the Property and all improvements thereon against all others.

And to expand a little further on Frank's cap-x comment, during the last recession there were plenty of apartments with 20% vacancy or more, simply because the ownership didn't have the resources to do the capital improvements necessary to attract tenants. 

Also sounds like you might not have the capital for a major improvement if you buy something that needs work?  

Take care of the property and focus on credit improvement. 

We'll likely throw money at it once someone moves out and we need to improve it to increase rents yet again. 

Since they have been really good renters and have at times been some home improvements themselves without asking me to reimburse,  what would be considered a reasonable and fair thing to do?

There are definitely homes under 140k and would rent really well, especially older homes in their improving downtown. 

On the other hand, investors often are the only ones putting money into renovating these properties which gradually improves neighborhoods over time.

Equity comes from improving the property, raising the rents, i.e., making it worth more than when you bought it.