I plan to build a duplex with approximated 800 sq ft units, 2 bd 2 baths each.

Could you please give the bank that you used for this, and approximately how much was your monthly payment?

I have not as of yet retrieved estimates for rehab of both houses. 

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Wondering if you have any rough estimates of to how much a fideicomiso cost...

I had just said that we expected it to be about $110,000 and they were like “No we need a formal estimate”.

They are going to write up a field estimate and are willing to (usually) pay you (the homeowner) or your preferred Contractor that money for the repair.  

Few companies I got from google search offer free estimates (e.g., US waterproofing, Everdry waterproofing) but they are all swamped with work and I can't get an appointment until mid-June. 

By our estimate, it should take us about 10-12 years to payoff all the properties with this snowball approach.

Never trust seller's ARV or rehab estimate.