I would not put a new motor on just to sell the boat. I would think that would be the way to loose the most amount of money on your boat. Five grand to get it going to sell it in the lower teens seems like your best move. I see for one for sale now with a 90 zuke that has been for sale for a while asking 17,000.
Selling the boat and trailer with a bad motor will likely move quickly. I have not seen many lappys sold as projects.
I would personally list it for sale as is and swing for the fences on the price. If you get a number you can live with let her fly. If not take the listing down