Motor size?

I've been thinking of buying this jon boat with a trailer and fish finder for a pretty reasonable price, only thing is is that it doesn't have a motor on it. I'm not really familiar with boat engines and all that, but it's a 12ft flat bottom and it's rhino lined on the outside, it's got a Hummingbird 586c HD fish finder. What size motor do you think I would need? I think a trolling motor would be too small but I'm not sure.

I have a 9.9 on mine. I ran it for years with a 4.5 and it was not bad. The boat and engine sat unused for 20 plus years in the barn, and seized up, since I didn't take care of it. I replaced it with the 9.9 last year when I decided to start running lines again. I have a 2 hp Johnson I have run on it as well, but it is painfully slow.