I bought my Gammill used--it's a 2006 Classic Plus with a 26" arm & 14' frame. I have to honestly say that I rarely do a king size quilt (who can afford that much fabric!), but it' is nice to have the larger frame to load quilts lenght-wise. that being said, I also don't think the larger throat is that useful either--who has arms that long!!?? I do think the height of the throat is important so that you can roll a longer quilt.
My friend with a 12" Gammill frame put her's diagonally in her room and since she was small, could easily walk around the one end and work on both sides. her quilting room was probably not a whole lot bigger than your's, so I'd suggest you measure diagonally and see what your space is there.
Having your frame on wheels is awesome! Especially since I got laminate flooring. Just make sure those wheels include good brakes on them.
As far as a saddle stool--I suggest you try out both one of them and also a drafting stool. I've had both knees replaced and found the saddle stool a little hard to get on due to my stability issues--the drafting stool doesn't require me to "throw" a leg over like the saddle stool did.
Good luck! I think you will really find that the long arm will help with those back problems that you experience with your sit-down and sandwiching for it. just make sure that whichever one you choose, that it's set up the correct height for you!
P.S. Long arms heads are so heavy that you won't likely be hauling it to the dealership---so don't worry too much about proximity to dealer, but rather responsiveness to your phone calls for tech support. We become our own techs on a day-to-day basis with tech support. Most send their techs out long distance and the calls are expensive--so rarer!