Combined payments are 2,158 on the new first PITI and 351.90 on the new second total $2,509Current rent is 1,000 downstairs and 1,500 upstairs.Both mortgages will mature summer of 2026 at which time the house will be free and clear.

After looking into the property further I realized it is part of an apartment complex that consist of ten identical fourplexes and all of them are empty.

This is identical to a HELOC.

Have you determined a beginning, middle, and maturity schedule for your intended real estate investments?

It would end up being mathematically identical, assuming everyone is doing their math correctly. 

Also, the UK is very much an exception to the rule as identity is checked in most European countries with national identity cards.

All four units are identical, and according to my research should rent for between $725-750 per month.

The tenant had disclosed he owned one mature boxer.  

FIRST establish your investor identity; what type of deals do you want to get involved in (flips, wholesales, buy & hold, rent to own, etc), what type of property do you want to invest in (single family homes, multi family, commercial, etc), what's your risk tolerance, what sort of returns do you expect to get, do you want to be hands on or make it as passive as possible, etc.SECOND pick a strong market based on it's ability to meet your investor identity and based on strong fundamentals like I mentioned above.THIRD put together your team in that market.

But, she won't be an official tenant you have to deal with.If they aren't mature enough to understand this situation, you may want to keep looking for another tenant.