I just got a couple of 'yaks from my parents. Free 'yaks are the best 'yaks
But seriously, the nickle and dime shit will add up. Roof racks could end up being $1000 on it's own (Yakima roller and saddle setup). Most people want to upgrade paddles, that's another $100 each, then you'll want to upgrade to a different seat type, and that could be $50-100 a piece.
That's after getting the kayak you want though. For a river as big as the fox, you won't want anything shorter than 10' IMO, and even then, that's pretty short. The longer the 'yak, the easier it's going to ride, but the harder it's going to be to turn. Then you get on width, the wider it is, the harder it is to get through the water. If you plan on big open bodies and big rivers, longer and medium width would probably be best. If you plan on going into a lot of little tributaries and portaging, shorter and wider would be better (but a pain on bigger water.)
IMO buy used. You can get some great deals on used, because people buy them with the best of intentions and never end up using them for whatever reason. My parents saved a couple grand by finding used stuff (that was basically new.)