I'm a little late getting in on this one,but here's my 2 cents. Take a serious look at a NDK (Nigel Dennis) Explorer. Many years back,when I was a new paddle,and I was ready to move into a serious boat,I looked at what all the harcore NSPN guys paddled. Almost everyone was in an Explorer. I've had mine for 8 years now,and never "outgrown" it. Good stability,very predictable secondary stability,great rough water boat,built practically indestructable(I put mine clear up out of the water on a rock,paddling full tilt-no damage). Plenty of room for camping trips,and I actually like having the day hatch for packing some items.
Down side of the boat-they can't build a seat worth a damn. Easy solution-cut it out and put a Redfish Kayak foam seat in-perfect!
Because the boat is popular,used boats are out there,most have some age,as people often paddle them till they can get a new Explorer.
Other worthwhile boats,some already mentioned,Quest,P&H Sirius,Current Design Gulfstream,Boreal Design has several nice boats like the Ellesmere.
There's so many good boats on the market now,it's mind boggling. Whatever you chose-do not buy a boat until you have paddled it. It amazes me how people walk in and buy boats they have never been in. There are some really pretty boats that don't paddle worth cr@p!
And while you're spending money,make a really good(translation-expensive) paddle a priority. It can make all the difference on a long trip,or in rough water.
A couple of good boat dealers
New England Small Craft
Billinton Sea Kayak
everything you ever wanted to know about kayaks-and more
Paddling.net
Kayak Clubs
North Shore Paddlers Network
Boston Sea Kayak Club (I'm a trip leader)
Good luck-and happy paddling!