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

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.

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.

If you can't find a PM, don't invest in that market, because a bad PM can ruin a good investment.

You keep digging, and it will ruin you! 

I like the mechanics garage one could also take massive cost seg on all that equipment.. and same could be done with doctors office.. i did that with a dermatologist a few years back and it has worked out great helped young doc walk right into a functioning practice without her having to go borrow hundreds of thousands to outfit a shell..

But when I bought property in Kelowna BC and got a CA bank loan the bank for sure wanted to create a relationship with me as a client.. brokers are just not as prevalent. in the US brokers for these types of loans are out there by the thousands some good some not so good.. when you find a good one you want to be loyal to them.