What you want to budget could determine some of this. Matching the year of your boat somewhat close would be ideal but its not always a perfect world. Find a dealer / technician you can trust to check spark, compression, and lower unit as a bare minimum. After you make your choice run good oil, keep the fuel fresh as possible and change the water impeller every couple of years. Good luck, 60-75 mercs from all those years you mentioned were pretty decent engines.