I just bought an Innova LA on a 12' frame last October. I couldn't be happier with it and would highly recommend it. I bought from Boersma's in McMinnville Oregon and if you call them, you might be happy. The owner has developed a frame as small as 5' made out of PVC-type pipes. You have to layer it yourself instead of using the rails to load the quilt backing, batting and top, but it would solve your space issue. He (Jack Boersma) says that Innova is going to pick up his frame and begin selling it next year, but he sells them himself in the meantime.
The lightening stitch is expensive ($4,000) but what it does is keep your stitches the same size as you move your hand slower or faster, or when you are stopping or starting back up. I wasn't going to get it, but my friend talked me into it pointing out that the appearance of your stitches is everything. I'm really happy with it.
My frame isn't on casters, but it's light enough that I can push the frame around on my bare wood floor. Unfortunately, like PaperPrincess said, I had them set it up in one spot in my family room, then I pushed it over to get it better lighting. Now it's not level and it's drifting when I let go of it. I haven't tried to re-level it yet but have got to do it before I do another quilt.
The Innova manufacturer is a family-business, and they are available 24/7/365 - even on Christmas Day - by phone to help you trouble shoot. I haven't had to call yet because I haven't had any problems.
As far as your back, they asked me how tall I was when I bought the machine and they set it up to my height. I have been able to work on it for hours without my back hurting. I can't say enough good things about this machine. I've been thrilled.
I also took a class at Boersma's about a year before as part of the process of looking at machines. That definitely helped. A class was included in the purchase price so I ended up taking an intermediate class after I bought it.
Good luck and have fun!