The thing that strikes me is “…lightweight so I can handle it by myself.” If you plan to pick it up and carry it yourself, or load/unload it using a truck, the 15 footer with a 61 inch beam is probably going to weigh about 225 lbs and be awkward to handle.
As to handling a 20 hp, most boats that size will, but check the capacity plate.
The 61 inch beam is a little narrow, but it’s nothing like a 48 inch beam (or thereabouts) jon boat as to stability.
I have a 14 foot v-bottom with a 65 inch beam. It weighs 250 lb and is rated for up to 30 hp. I can stand up in it with some confidence, but it’s still a small boat.