duckbum,
Where you from in Idaho? North or South? When you say "the big river" you could mean the Snake or the Pend Oreille, Maybe the WA Snake or Columbia Rivers.
Lowe makes a lot of boat for the money. If you are in N. Idaho you can't beat the Lowe deals at White Elephant in Spokane. If you are up north and want to see what a custom duck boat looks like I would be glad to show you my GatorTrax / HyperDrive combo. I run it on big and small water alike up here in the Panhandle.
If you are down south and running the Snake, you are probably thinking jet. If you go jet, make sure you get a dealer that can set it up properly. Or consider a mud motor. They are run on the Snake, although an 18 x 60 is an awful big boat for a mudmotor.