Depending on your state, you may be able to register the trailer with just the registration. Here in NY some trailers are transferable, just have the owner sign the back of the registration card, take it to the DMV along with Bill Of Sale and a few $$ later its in your name.
Looks like a fun project. IF it were mine, I'd ditch the front railing as it dates the boat horribly, paint the window channels, refresh the interior and two tone (or add graphics) to the hull side. I definitely see potential there. As for motors...I'm partial to Honda but my cousin and uncle have older yammies and they haven't had any issues that I'm aware of.