I also like the Remix a lot. Nice hull design and the liquidlogic outfitting is the best in the business. But as others have mentioned, all the boats you listed would work for the LY. My advice is not to worry about "displacement" versus "planing" at this stage. Start with something that fits you well and you feel comfortable in. Also, if you've started to work on your roll (and you should be!), and one or more of the boats that you've tried feels easier to roll, that's a good selling point for a beginner too.
Otherwise, your plan seems to be a good one. The lower yough certainly deserves your respect, but assuming you'll get in a fair amount of practice during the next season, it could very well be an end-of-summer goal. It's pretty forgiving of newbie mistakes (I speak from experience), particularly if you're in the company of good paddlers who know the river well.
Oh, and just fyi, there are actually three sections of the Yough. In addition to the lower and middle there's the upper, but that's solid class IV-V stuff.
Classic Freestanding Rack
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