What's the highest success percentage method to find a previous tenant's new address for this? 

It has been a huge point of concern for large investors as they want to hedge risk and co-living is a complete unknown. 

do they have any concerns?

Given the scope of your work, a simple plumbing leak would not concern me, it's a $200 issue at the most and ideally, it's just another line item when you do rough in plumbing for the kitchen.Being in a Milwaukee suburb, I am guessing it's a 1960s ranch, probably with galvanized plumbing.

I'm more concerned about the drip-edge/ flashing on top of the roof.

My posts aren't specifically about Rent to Retirement, rather the concept more generally.I am also not concerned with the new construction turn key model, its very different than the lower tier/entry level SFH's and not relevant.

We've also advised this method to our investor clients that borrow from us.

Happy to help answer any questions about the flight path, but in general there aren’t many areas where that is a concern.

Definitely do more research on the BRRRR method as that sounds like what you're trying to do.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.