To be honest the best thing you could do is join the club and use the club boats for a while. The advantage of this is that there is a variety of boats available, river, sea and even a playboat. And club members are a friendly lot who have a wide variety of makes and models themselves.
If your thinking of expanding into whitewater, "...more challanging stretches" as you put it, bear in mind that river and sea kayaks are totally different. Sea kayaks are generally 15 feet +, whereas most river kayaks are 8 feet (ish).
There is so much variety out there between kayaks, paddles, gear etc its mindboggling, at the very least, come along and talk to people on thursday night to get a feel for whats good and what isn't.
If you haven't any experience, I can't stress enough the importance of doing a course. To build up paddling skills, as much as to know what to do if something goes wrong on the water (a matter of when, not if). The clubs level 2 course is the cheapest available, with ongoing skills development supplied on thursday nights and club outings.