Have you considered building traditionally? The main benefit for you being more room for the legs. Access to stowage is also better. Might be fun to learn a new building method, as well. You could build a Nikumi traditionally, or just the cockpit, for the extra legroom.

Sea Riders are very nice boats.

As for rolling, WW instructors will likely teach the C to C, which is good for protecting your face from rocks while underwater. It isn't a very relaxed roll . . . .
For sea kayaking, I prefer a nice, calm layback. Learning one might be easier. Check out the Standard Greenland Roll here: http://www.kayakways.net/gallery/videos/ He makes it look truly effortless.

Regardless, a good fitting dive mask, or at least goggles and a nose clip, will make learning far less stressful. And rolling in a WW boat will carry over to your sea kayaks, so no need to forego the class if the pool is too small for your Sea Rider.

Cheers -
Dave