I would have concerns that even after spending the 5K to clean up that there might be residual chemicals or other contaminants around.

For a year and a half, we watched these two finally give into the fact that they had to clean up.

In the right circumstances, depleting your retirement plan to make investments in real estate can make long term sense - but you absolutely want to weigh the pros and cons specific to your situation with a professional.You put money into the 401k pre-tax.  

That depletes their savings. 

If a property has a 13% cap rate, it's likely it that will be reflected in the price and if it's a building in good condition, tenants in place, that will be of interest to many investors, which means the seller will not be a distressed seller, IMO.You'r probably looking for a run down motel that can have some clean up, run the ladies off and rent the units out as efficiencies.....or some similar property!

And not all tenants clean up pet pee as it happens.Still, a no pet property is a no pet property.

Hire companies to do all the work besides normal cleanup.3.

What concerns me about all of these is this: when the end-buyer defaults and trashes the place, even though they were a "buyer" and not a renter (it will happen, ask me how I know), who is responsible for the clean-up and repairs?

You will need to furnish these places, have them cleaned daily or have some sort of agreement with the tenant as to cleaning schedule for the unit, maybe they are responsible and you just have to clean up when they leave.  

So, if the tenant doesn't clean up after their pet, I would charge them to do it for them.