A couple of thoughts for you: First, you are in a great position in having 20% for a capital contribution … plus, you have obviously cranked the numbers and feel that you can comfortably fully amortize the remaining 80%, over 10 years. This presents an opportunity for both you and the seller, if he would agree to a seller takeback mortgage structured at something like a fixed rate of 4.75% and fully amortized over 10 years. If you borrow the 80% from your bank, the seller will receive 100% of the selling price at settlement, likely without an investment alternative that can earn 4.75% for him (check the current Certificate of Deposit rates being offered by financial institutions) … so providing financing for you would provide him with the 20% equity position to make the deal less risky (he would also, of course, hold a mortgage on the building), while simultaneously earning him a higher rate on the remaining balance than he could earn with a bank or other financial institution. The “win-win” for you would be the fixed rate mortgage.
There’s no harm in asking if this concept might be attractive for him. An attorney can draw up the needed mortgage documents, and record them for the holder.