I also had the Mercury / Tohatsu 3.5 HP on my SSC 18. Was nice to have some extra power in some conditions. Still only 19kg and Tohatsu also offered a sailing propeller for lower boat speeds. Depends on your sailing area. Less might work out fine for you, more than 3.5 HP would probably be too big and too heavy. I chose against electric engines since they seemed to be more like toys and you can not leave them attached permanently on the boat. But might also work on a small lake and if you don't care stowing it away every time. Pro electric is definitely the silence while running under engine.