I estimate after loan payment, taxes, insurance, Free Cash Flow will be around $1350.

There doesn't seem to be a GFE, Good Faith Estimate or TIL, truth in lending statement required.

I’d get The Book on Flipping Houses and The Book on Estimating Rehab Costs by J Scott for flipping.If you haven’t already you need to find a your local REIA or other networking group and start going to those.

I had trouble finding a vendor due to a harsh winter last year. so I approached a vendor that had given me an estimate back in April 2021 (when I purchased the property). 

So to come up with my offer on a house to present to a cash buyer, I use the following equation:ARV x Investment Percentage - (Renovation Estimate) = Cash Buyer OfferSo I'm trying to figure out how people come up with the investment % the use in the equation.

Advertised Cap rate is 12.5%, although that will change as I'll have to add management costs into the deal (estimating 8% of total annual rent for this, please let me know if it should be higher than this). 

ServPro did complete their work by the first week in October, and I was able to have my GC provide me with an estimate for all of the put-back.

They are all projections of estimated returns. 

The property has space for approximately 27 units.And I have no idea how to estimate monthly operating costs.

Then let the potential buyer know the situation and that the estimated buy-out is already included in the numbers you've presented.