-- Last Updated: Oct-10-12 10:53 PM EST --
To be honest, you can run that in *any* boat. That said, if you primarily want to run downriver, get some lines, catch some eddies - get a river runner: Diesel, Mamba, Zen, Remix (not the XP barges, the regular Remixes), etc. They are all good - get the one that feels right for you. You are probably the medium size (or one below the largest).
If you want more play, get something lower volume or a river/play boat. FunRunner, Locki, Axiom, RPM, etc.
I've only run the LY a few times - a couple of times with my WaveSport Fuse 64 and once in my new Axiom 8.5. So I can't say I "know" the river... But at the weekend levels of 1.8 -2.2 or so, it is a nice step-up for an aspiring class III paddler (and you can find lines that are hard - you can do class IV moves in class III water, as they say). My 2 kayaks are very different boats and offer different experiences on the same river. The shorter chubbier Fuse can surf the smaller and steeper waves, spins on a dime, plays all the features. Yet it has enough of a volume and length to be stable without requiring too much attention. The longer and much faster Axiom lets you enjoy the longer faster waves and do some lines that require speed, can attain to go up and repeat some runs, etc., but can't surf the steepest waves like shorter boats can.
A full-on river runner will be more reassuring than either of these, so probably a better choice for you to start and fewer chances to flip over (which you will, in any boat).
A creeker will be an overkill - too much boat for no good reason on a river like that. Heavier, not so playful... But will do the job of getting you down just fine and probably be just as stable as the big river runners if not more so.
I'd say, demo a few boats and get something in your size that feel scomfortable. It probably won't be your last boat, so don't sweat it too much if the price is right and it is one of the well regarded river runners.
For the flatter sections, none of the above will be particularly enjoyable though. But on the rapids is where the fun is with these boats.
On the other hand, if you just want to get through the rapids and enjoy the longer flatter sections, looks for something longer and with retractable skeg...
As for displacement vs. planing hull, there are advantages to each. Having a planning hull does not mean you have to have super sharp edges or vertical sides. It just means most of the bottom is wide and flat (as opposed to rounded). With a flat bottom you can lift an edge and let the current just go under with you and your boat barely feeling it. With a rounded bottom you can't quite do that - you will get pushed around more. The rounded bottom hulls are generally more hydrodynamic so they tend to be faster and go smoother with less slap through water and waves. But are you racing? I think for most people that are just starting, a flatter bottom boat with forgiving sides (chines) is the way to go - they have a more defined stability when edged, are less affected by eddylines and cross-currents, are generally easier to spin around (more maneuverable), yet when put hard on edge (if they have a good edge, that is) they can hold a good line and track well to keep you pointed where you want them to go.
As for the high knee bumps, yes, a higher angle stroke will cure the issue of your knuckles hitting the sides. It's a personal preference. I prefer higher and closer knee position compared to wider and lower. You can test for yourself and prove to yourself that you actually have more control of your balance and the boat if your knees are not too fat out to the sides splayed out like a frog (there are exercises to do on land to show this to you). But comfort is more important - make sure you test with paddling shoes as that makes a huge difference if you fit or not (especially if you have large feet and long legs).
And think of a nice lighter paddle - makes a big difference compared to a heavy low-end paddle...