And my 2 cents on the boat length issue...
Also consider WHO you paddle with. Going into my first challenge, I had every intention of paddling the majority of the way to Key Largo alone. Partly because I didn't really know anyone, but mostly because I'm an introvert and the thought of having company most of the time was completely unappealing. I was truly excited for 8 days of alone time. My boat was a P&H Sirius, 17' and no rudder. I hadn't paddled a ruddered boat in years and didn't feel a need for one. Even though I actually only wetted out about 15'8" of the boat, I didn't really feel too slow and happily cruised around 3.5 kts (empty).
Then reality hit. During the first half of the first day, I was very happy to join up with someone for a bit. I would have loved some company crossing Tampa Bay for my first time! What really happens is that you paddle with someone for a little while but eventually someone's pace changes a bit and you're back to paddling solo. Someone stops for a break, the other doesn't and you're back to paddling solo.
By the time I got to the big bridge in Sarasota it was early afternoon on a beautiful day. The boat traffic was insane and the busiest part of the bay was 3' slop in every direction. When I saw a nice yellow Epic slide up into my periphery, I was ecstatic! Never thought I'd be so happy to see another kayak! As it turns out, our paddling style synced very well and we continued to paddle together for the rest of the EC. We hit it off so well that we live together now!
I had always been happy with my Sirius, at least as far as speed went. There was just no way that I would ever be able to keep up with Kurt in his Epic though. Between the huge difference in wetted water line and no rudder, he would always have to hold back so that I could keep up. It's a rotten feeling knowing that you're holding someone back even if they're genuinely happy to do so. It's also a rotten feeling when the boat you're traveling with is *just* able to beat the current and you're *just* not able to. After the EC when we paddled together we would often trade boats. Sure enough, he couldn't keep up with me when we traded.
So at this point I knew that if we were going to keep paddling together we needed to have comparable boats. The thing was, I really like squirrely lively little boats. The Epic was nice and roomy and fast and more stable than my Sirius. And it bored the hell out of me.
What I finally concluded was that it's a matter of the right tool for the job. If I'm paddling solo I'm happy with a shorter water line and no rudder whether it's a day trip or the EC. But if I want to paddle with longer, ruddered boats, I'm going to paddle a longer, ruddered boat.
I'm not a powerful paddler. My technique is far from perfect. I definitely put a lot less effort into maintaining 4kts cruising in my Stellar than in the Sirius. You WILL get something from a longer boat. The question is, do you NEED it? People do the EC in 16' boats all the time and do it well. You certainly don't need a rudder. But I will say that when you're exhausted (not "tired", but actually "exhausted") and it's dark and the water is sloppy, you will find yourself reluctant to edge agressively in order to control the boat.
Think about what you will want in a boat when you are exhausted, cold, hungry, weathercocked, and just generally have the heebie jeebies about the situation you're in.
Lots to think about and only part of the puzzle. Sorry that there isn't an answer there, it's just my particular journey in choosing a boat. I love my Stellar and she treated me like a Queen this year in the Washing Machine of Death. I am certain I would have been totally unhappy out there in my Sirius. My Stellar takes me far, my Sirius makes me grin and giggle :-)