There's a reason Baltimore prices are what they are, even though it's a coastal city.

Tulum is a magical location from the white sand to the jungle behind. 

As others have mentioned, 100K in some markets is doable but in most coastal, appreciation markets, it will not be sufficient to lend in a safe manner, i.e. with a large equity buffer to protect your principal investment.

My partner will be general contracting at no cost, although he will be paying a few key 1099’s and contractors .

So for effectively $5-600/mo plus their mortgage payment they were able to secure their dream home and eventual coastal STR.

Let’s say FHA loans in the high 4%’s and conventional in the high 5%’s? 

What coastal area?

I head my hand in the sand then so I don't know. 

Any property in the major coastal cities leveraged at 75% ltv would be cash flow negative. 

I ran into a small company that had gone to a guru event back in early 2000s and were sold on the sub too then lease option sand sandwich type deal and did not care about equity just about cash flow.