@Robert GonzalezMost any DSCR loan is going to be chalked full of docs where you agree to NOT occupy the property while under the terms of the loan you are signing docs for. 

Prepayment penalties- usually 1-5 year terms.

I recently purchased a Short Term rental property and plan to self manage the property.

Over the long term.  

The ADU would be utilized as a short term vacation rental. 

What terms is he offering (monthly payments, amortization, balloon, etc.?

-Seller Expectations: Ensure the seller knows and understands what to expect regarding terms and timeline-many sellers are unfamiliar with seller financing.

It may be possible for the homeowner to talk with their lender's workout department and negotiate terms to pay off the arrears, penalties and late fees on a new schedule. 

I've read numerous guides and watched many YouTube videos, so I have a good theoretical understanding, but I'm still unsure about the best way to start investing in U.S. real estate.My plan is to begin with long-term rental properties and then eventually move into flipping and the BRRR (Buy, Rehab, Rent, Refinance, Repeat) strategy.

I intend to get a single family residence, add an ADU, live myself in the residence and rent out the ADU as a studio apartment long term.I have checked my city's ADU laws and long term renting is permitted.Question : can I obtain the REP status while maintaining the ADU while it is in my primary residence?