I will spare you advice about taking lessons etc. since thatâs not what youâre asking.
Speed and stability:
You are wise to consider speed. Not (I hope) out of concern for keeping up with the group; if your group doesnât adjust its pace and trip plans to the speed of its slowest paddlers, consider going with a different group. But faster does mean less effort to cover distance, which is critical.
Speed depends on the length of the waterline, NOT the overall length of the boat. (A 17â boat with a lot of rocker may have only a 15â waterline.)
How the boat is loaded also changes the waterline AND the overall effective hull shape (more gear = boat rides lower), and therefore can also potentially affect the speed, stability and/or tracking. A boat that is designed to be loaded may actually paddle faster than when it is unloaded. So test drive a loaded boat if you can.
Stability per se doesnât seem more of an issue for a longer trip (unless it negatively affects your comfort); but obviously looms much larger if you are paddling on open water.
Initial stability (âtippinessâ in flat water/resistance to putting it on edge) vs. secondary stability (resistance to going from on edge to upside down) are two separate considerations. Counterintuitively, a boat will behave better in wavey/choppy water if it has LOWER initial stability because it wonât be tipped as much by the waves (see KayakHipsterâs excellent YT video on boat stability). But low initial and high secondary stability are not mutually exclusive. Low initial + high secondary = the ideal combination for confidence in choppy water.
I paddle an 18â P&H Cetus MV (Iâm 6â, 190 lb) which I find an excellent rough water boat, and with careful planning was able to do an 8 day trip in remote coastal Maine where we had to carry drinking water. On the roughest day, with 2 1/2â broadside breaking waves, I was having fun where others may have been nervous.
Do consider weight though, if you will be dragging your laden boat up and down the beach every day. Very broadly speaking, Brit boats (like P&H) tend to have heavier lay-ups for the same material.
Tracking
Yes, long, straight, and narrow will track better (and paddle faster) than short, rockered and wide, but there are always tradeoffs, especially maneuvarability and capacity. Not to dive into the debate about skegs vs. rudders, either will help any boat track better if used correctly. I find that a rudder provides greater support for tracking. One downside of a rudder is it does create some drag, especially when deployed at an extreme angle to compensate for strong broadside winds.
Carrying capacity
Obviously, a bigger boat (longer, wider, higher deck) will give you more room, but has trade-offs. If youâve ever done any backpacking, you could probably make just about any touring kayak of 16â or more do - but the nice thing about kayak tripping is the luxury to carry more. Another consideration about skeg vs. rudder is that the skeg well takes up room inside your rear hatch, not an issue with a rudder. But because a rudder lies on top of the deck, while it improves tracking when deployed, in wind it can negatively impact tracking when itâs up.
Seating & Ergonomics
Very personal. One thing I love about P&H is they use a fully molded seat that I find tips my pelvis back and helps maintain good posture. I have found this much more helpful than padding or a high seat back (the latter actually discourages a good, active paddling posture).
Legroom should be sufficient to allow you to maintain a relaxed froggy position while still in loose contact with your foot pegs/rudder pedals and thigh braces, but still allow you to actively engage both of these without having to reach. Also make sure the cockpit length is long enough that you can wet exit/re-enter the boat easily.
As you can see from all these posts, there is a LOT to consider. Whether you decide to buy, rent, beg, borrow or steal a boat for your trip, I strongly recommend that you borrow or rent as many different boats as you can to try them out on longer days/weekend trips first.
Good luck, and post some pictures!