I am not sure about a rent by the room comparison vs their standard single family or small multifamily estimate, so I would check with potential lenders to see what they will consider.

Personally, I have very high standards.

If you can't do the standard 20% for rentals, look at duplexes where you will live in half and rent the other half. 

However you also have to consider what you're going to do when your tenants move out at the end of their one year lease, and how long it may take for you to find new tenants that are suitable to your standards. 

Old style buildings with lobbies and internal hallways.

But that is like in any international destination at the moment because of Covid and the Riviera Maya (including Cancun) is probably one of the least affected areas as it is one of those that accept visitors and on of the very few that accepts American visitors.

When I suggested adding something specific in the offer that says “buyer can back out if appraised value is less than X”, they suggested to deal with this after the offer stage and at attorneys review, and use the brokerage’s standard/unmodified contract until we agree on the price.Am I overthinking this?

There are three in particular that are related to the return on real estate investments, and each has its own unique meaning and purpose.They include CoC, which stands for Cash-on-Cash Return, IRR, which is an abbreviation for Internal Rate of Return, and Average Annual Return, which is self-explanatory.

Is there a standard set of terms?

All fairly standard, all fairly mediocre.