What are some legal factors (Mexican laws or taxation) to take into consideration? 

I can build cheaper than I could replace but there is a time and stress factor to consider.

The key factor is the amount paid and the nature of the payment, not the payment method.

Sounds like they could have different PPP or fees that go along with refinancing with a different lender. 

Even with a 25% fee, you're cash flowing better than a long term rental, so the fee should not deter anyone! 

The good news is sometimes you don’t need to pay the thousands in attorney fees.

I think your best approach is to pretend you are the investor yourself, calculate build cost (mortgage with interest) + Maint / cap expenses/ management fees according to your zoning and then compare that to the average rent in that neighborhood.

BP can keep 10% as a service fee.  

Its up to you as the builder to negotiate a fee that your happy to pay.. and Frankly market conditions dictate commission in hot markets one might be able to just stick a sign in the yard and do yourself.. still pretty tough or just put them on MLS and negotiate the buyers agents commish like the new rules dictate..

BUT for certain rural areas it's worth it's weight in gold because of the reliability factor vs an unreliable or expensive existing utility structure.