Your specific variable expense assumptions are wrong, BUT you stumbled into them being correct overall.Also, you are approaching this correctly - entering in the numbers to generate a purchase price that meets your metrics:)

He was happy to allow me to rent out nightly because it actually helps keep his property in better shape than a long term renter (having a cleaner in there every turnover, short stay guests always let you know when something isn't working, etc) but didn't want to hassle with worrying about a percentage of income/management, etc.

To determine price range, the first step is always figuring our how much you are pre-approved for; and that can vary significantly when you are looking at multi-unit properties because of the additional variable of rental income that can be included in qualifying.

There is a lot of variability in this answer and, like all things in real estate, it simply depends. 

With the new commission rules you can stay flexible with an offer of buyer agent commission, rather than having to commit to a certain percentage going to the buyer agent upfront. 

In Costa Rica, property condition is a constant variable, far more critical to maintain than the USA. 

If land values are increasing significantly, you can back-end load the seller financing for a percentage of the profit when you sell.To add to prior comment, how many septic tanks do you have?

It even states that the lease “will repeat itself” but doesn’t specify terms like figures or percentages for rent increases.

Fixed at 3.5 for ten years variable after that up to 10.50 with 2 point max every two years.

To Sum it up, its a good idea if you can exceed the cost of capital (higher rate than your cost of debt) with what you earn on your heloc/debt you pull out and invest.The question is how much gains above is enough to substantiate the risk of your variable interest rate, well thats up to your personal investing criteria.