I would recommend getting a couple DSCR quotes, a couple conventional quotes if you want, and go with the numbers that help you sleep best at night. 

At that point do whatever makes you sleep well at night.

Obtain estimates for planned renovations and confirm if the allocated budget is sufficient.

However, that's not sufficient.

That will completely break my self-sufficiency test. 

After the seller financing is paid off in 4 more years, this property should create a 15% cash on cash return: airbnb.com/h/majesticlodgecostaricaWe are currently also expanding our beach house, which is already a six bedroom home that can sleep up to 28 people.

Yes, there is a chance the payment could be rejected for non-sufficient funds on the 19th, but I'm managing 400 rentals and it's pretty rare and the Tenants make good on it 99.9% of the time.

I have found that the best answer for YOU is what will allow you to sleep well on your pillow at night without thinking too much about it.

Being that you already have property management experience and feel confident in your ability to manage a single-family or multifamily property, it really comes down to your financial position.If you have a good amount of cash available to put a down payment towards the purchase of a property and maintain sufficient cash reserves (or if you have a partner you can work with who does), then there's nothing wrong starting off with a multifamily property. 

Your primary insurance policy with $300,000 in liability coverage should be sufficient in 99.999% of all lawsuits.5.