It is now paid for, is worth triple my purchase price, and provides about $60K a year in rents.  

Work with your attorney for what is needed to insure all subcontractors get paid by the General Contractor.Make sure the insurance covers Construction period and also materials.Talk with your Commerical insurance agent and they should address the above questions with you.If this is an older facility make sure your engineering firm has covered Fire Suppression systems and any Storm pond or drainage systems. 

I had a chat with them on the phone as i was curious about the funding.. but came to the conclusion that the down payment ( the client made to buy) was probably what they paid for the property and the seller carry back note was the profit.. and then the seller carry back note is sold to yet another investor.. who is investing for coupon money.   

Private loan that I paid when the sale was finalized.

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If I had to guess I would say your money could “develop” 1-2 houses in Staten Island but in the right market you could easily get up to 12 or possibly more good units fully paid for.

Private loan that I paid when the sale was finalized.

And lets say you then rehab it only to find out there is bad title and you make a title claim.. if there is no loan on it your only going to get 50k from title insurance and you will lose the 50k you paid for the assignment and any money you put into rehab. 

However, if we paid for the house mostly in cash, does it make sense to refinance and incur a high monthly mortgage payment? 

I was able to replenish emergency fund money I had to dip into for furnishing, was able to pay off a hot tub, was able to replace 2 garage doors and paid for a larger repair this year.