Then look on VRBO and Airbnb and guesstimate rates and occupancy for your area on comparable properties.Put all this in a spreadsheet and let it do the math.

This $500,000 property has a 30 year mortgage with 25 year left at a 3% interest rate.

. - Buying a larger home for growing family with evidence of increase in dependents and 25% equity in current homeCheck in with @Joseph Chiofalo, he's a great friend and top lender nationwide for 20+ years.

That being said, it's tough to get over the relatively high rate of return for STRs when you are comparing them to LTR. 

Add a short statement at the end outlining whatever you are changing, have all parties date and sign, and you are done.Example: "Landlord and Tenant agree to extend this lease for an additional 12 months with a new termination date of 12/31/2025 and a new rent rate of $1,950.00 per month."

I'm looking for a PM group that has a reasonable monthly rate, manages tenant selection well, addresses property issues with quality, and conscientiously and actively turns properties when vacant. 

Depending on how you are planning to pay your friend  the profit out of the transaction.

Also, in our experience, 95+% of agents have no clue how to calculate ROI, Cash-on-Cash, Cap Rate, etc., so what are they really going to help an investor with?

It seems like you have a reasonable down payment available so offer them a reasonable interest rate. 

Not sure what the laws are in ME, but in Georgia a first position foreclosure will wipe out the 2nd position lien making them riskier which commands a higher interest rate percentage.