@Trevor DeSimone
Most of these have been mentioned on your thread in parts, and I'll add a little.
You are underwriting the borrower and the property.
Underwriting the borrower is art as much as science. You'll want to understand the borrower's likelihood to complete the project and pay you back. So, look for experience, size of the rehab, market conditions, and borrower credit strength. A $200K loan with $190K acquisition and $10K rehab is totally different than a $200K loan with $10K acquisition and $190K rehab.
Underwriting the property.
1. LTV
2. Deed of Trust / Mortgage
3. Be listed as mortgagee / first payee (not additional insured) on insurance policy.
4. Title insurance. Get lender's title insurance.
5. Promissory Note
6. Detailed scope of work / photos
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Borrower's GC. If it's a large project, you'll want to understand the skill set, experience and license level of the GC, too. Check out other work that is similar to the subject project that the person has done.
Hope this helps.