And some semblance of knowledge on owning and running rentals.

Its ALL about knowledge All the best 

I want to help them from my heart, but this is new to me, and I am not knowledgeable enough to help their needs in every single steps, but at the same time, they may know how unknowledgeable I am.

I'd seek out a local nursery (not box store) that are knowledgeable in the local flora. 

In Punta Cana, you'll have tho contend with the fact that most visitors want to stay in all-inclusive resorts and you can't compete against that while, in Tulum, you might have to contend with overbuilding of epic proportions and the ensuing competition.

They have been staying there overnight consistently.

Lindel's Plumbing. 317-401-0643I'm not sure of his current schedule, especially if its as short of a notice as tomorrow, but if all else fails, he is extremely knowledgeable and may be able to point you to another reliable plumber. 

Given your lack of experience, you need to stay away from D/C- neighborhoods and develop very strong WRITTEN vetting criteria and know how to implement it (not allowed to ask criminal history early in the process, etc.), AND implement it rigorously - sob stories need not apply.There is a very high failure rate among new landlords; you’ll find out why.

We plan to stay abroad, but want to start investing back home in the US.

Generally speaking, MTR/STR guests make less damage/wear on the property due to the shorter length of stays.