Off hand, I would say that a 10 foot table is not going to fit very easily. I have 10' frame on my HandiQuilter Avante. I just went down and measured it and from one edge to the ends of the crank handles is about 10'7". If you want to do pantos, you will need to work from the back of the machine and you won't be able to do that with only 1' to spare. It will also be difficult to load and advance quilts without being able to get easy access to the roller handles. They usually suggest 3' on the ends. How wide is the doorway? Maybe you could install it so the end with the handles is in the doorway? Will you still have room to get in?
As far as going bigger, with a bad back, I wouldn't look at a head longer than 20". Remember that you will be stretching forward the 20" to quilt the entire area.
I'm also a Viking gal but I also would stick with a longarm from a dedicated long arm manufacturer.
I think it's a very wise decision to take a longarm class first. That will give you a good idea of what you are getting into.