I took a look around the net for 20hp jet drives and it looks like Mercury sells one. I would get a jet for that because nothing ruins a day like whacking a prop when you're running a river. As long as you don't get into tide water too much or with heavy chop the jet will get you out and back. Speaking of chop with the flat bottom having a prop won't smooth that out anyway.
I would do the Merc 20 because if you use it on any fair size river you're going to appreciate the power and with that intake vs. a prop you will have less trouble. You just have to trust me on the jet thing, I've broke enough props and shear pins to really appreciate a jet drive
With a 20, I'd get with them for a custom package, you'll want the seat about mid ship and use an extension tiller to drive. With the big motor and a driver at the rear trim could be a problem. Besides trim issues I never liked having to look over the bow from a stern seat to spot obstacles.