I think you can ride what you like. It's a long ride so whatever you feel comfortable riding. Personaly, i'm going to do it on my rigid p7. I have been riding her a lot, and my wrists/arms don't seem to suffer to much. Plus, I have had loads of adventures on the rc200 over the years, so it's the new (to me) bikes turn for an adventure.
In terms of numbers, I agree a larger group would be problematic. Both in terms of accoms, and also more of a chance of mechanicals, punctures etc.
If there is massive interest maybe it could be done in two groups (different days)? Just a thought.....
I vote we leave it until after xmas to see how many are interested, then make a decision as to what to do.