I am originally from Ontario just across the Thousand Islands bridge from Watertown.

Its thousands of dollars a year to maintain a license.

They ruined one of my Jacksonville properties.

They ruined one of my Jacksonville properties.

Hello Misael, I would do a thorough market research to verify that you can get the rent increase you want, you might find out that the market is oversaturated with rentals.If you are trying to raise the rent $100/mo, 1 month of vacancy ruins your rent increase for the whole year.I would list the property for lease 45 days before your tenant's lease expires, if you have your property priced right you should get some applicants before your lady moves out.

Now, here's a lower-risk, likely value-add Subject To Deal: Buyer is well capitalized, bringing hundreds of thousands of dollars down.

Any tips to keep my good relationship with the owner and not ruin it in the future because of this business?

I know many investors from overseas that have lost 100s of thousands of dollars. 

The last thing you want to do is spend a few thousand planting clusia, and then have to spend another couple thousand because it violates an ordinance.  

Making an extra payment each month, or even once a year, will accelerate your loan paydown and could save you tens of thousands and reduce the time to pay off the home.