Many visitors to Tulum live in a crammed small apartment in the likes of New York or Toronto and the last thing they want is to spend their holidays in a very small place.

Black looks good too, but in a small foyer it might make it feel even smaller, and darker.Have an "A" and a "B" prominently on the doors, so visitors are not knocking on the wrong door disturbing the other renter.Might be a good time to upgrade the foyer flooring to non-slip and the overhead lighting if they need it.Make the entry experience a good one for the renters.

Count me in if you guys would be willing to accommodate remote visitors. 

Building Rules.Tenant will comply with the rules of the Building adopted and altered by Landlord from time to time and will cause all of its agents, employees, invitees and visitors to do so; all changes to such rules will be sent by Landlord to Tenant in writing.

my husband is saying $50K if we want nice finishes) to make it a plush and attractive unit to overnight visitors. 

This makes Brea a popular destination for both residents and visitors alike, which can help to drive up property values over time.

@Rossitza IovtchevaThere are many other reasons to invest in Mexico and, in my humble opinion, it's the best time ever to do it for a slew of reasons on top of those you mentioned, the main ones being a rising middle class, a booming tourism industry, a growing number of visitors beyond tourism (remote workers, expats and retirees), the best possible regulatory climate for short-term rentals, much lower prices and costs, diversification and asset protection.

Since you plav on renting it out, you now are aware there is an existing problem (electrically) and if something were to happen to a tenant or visitor, you could be found negligable and therefore liable for damages.

I'm not sure how you can limit frequency of visitors, or time, etc.

You should be able to eliminate the problem in a few weeks, then just keep a few glue traps out incase you get an occasional "Visitor".