Keep an eye on the "RIBs for sale in North America" section of the North American forum. Every so often, used zodiac FC-470s show up at decent prices and in very good condition. For even better performance in rough conditions a catamaran hulled foldable inflatable boat (such as a Gemini Dive 470 or 530) will provide a substantial improvement over the futura hull design of the FC-470.
4.7m -5.3m is an excellent choice of length for foldable inflatable boats because they are so versatile (though they need to be trailered to be practical for one person to use). At this length, they can readily handle rough open water conditions at respectable speeds, though typically not at as fast a speed as can be handled large, heavier RIB. However you can also run these foldable inflatable boats up rapid strewn river systems where you'd never be able to get a similarly sized RIB into.
As for engines, in addition to the 4 strokes, it is also worth having a look at the direct injected 2 strokes such as the Evinrude ETECs and the Tohatsu/Nissan TLDI series, which meet the most stringent of emissions standards.