Welcome to Just Answer. I am here to help you resolve your tax and finance concerns. Please feel free to ask anytime you need extra help.
Perhaps the best concept you can offer the lender is a participation for his loan. In other words for a lower interest rate you offer a percentage of the profit on the sale of the units. Given the track record you can show this lender it may be a very enticing offer. Banks are not paying him very much right now so a low interest rate can be simply 2-3% more than he will get at a local bank. Yet this, combined with profit participation could make this far more rewarding to him. Yet it will only be so if you do well and then the larger profit will also be yours. The key to negotiating with a lender is to keep the structure simple. It leaves less to mentally digest, is easier to monitor from both sides of the deal, and offers rewards for a profitable effort. However, any agreement between you should be in writing. You will need an attorney to draw up the documents.