Definitely hire a real estate attorney to draft or review any seller financing agreement to protect your interests.Ask the real estate attorney how to handle the POA situation...can you talk with any of the other property owners who are short term renting out their properties to see what/if they have any issues? 

I have called lenders, and they say either they're renting them out, or they don't have any REO.Since lenders can often be slow to respond, I stopped looking at them. 

What we've done is use the BRRRR (Buy, Rehab, Rent, Refinance, Repeat) method.

The other needs less rehab with houses selling for around 100k or renting for $600-$800. 

Do people just wait for tenants to give them checks/transfer rent or do we need to send a invoice every month to tenant for rent payment?

These would increase our equity and business value tremendously How did you finance this deal?

You would need to notify them prior to renting the place or when it’s time for renewal so that they are aware.

If I hired a PM that would also take care of collections (among other things) would I still need a service like rent redi? 

When rates inevitably go down, prices will go up so people buying now will be able refi thus increasing cashflow while also getting appreciation.

The short term basis of this would be to AirBnB or VRBO your house - where they rent it as a long term rental from you, then lease it out on a nightly basis for the same purpose - to keep the difference for themselves.