The inflatable will have a rating for its maximum recomended horsepower on the transom. Some will also have a range of recomended power for an outboard. You're probably looking at about a 25-30hp jet to push you and a partner around in a 12ft inflatable.
I'd talk to another dealer. The one you've spoken with should have been able to tell you what size outboard will work for the boat you're looking at. Jets work well to push around inflatables for river running.
Look at the boat manufacture's web sites. They will list the size outboard you should use for any particular model.