I hear you about not being able to crawl around on the floor! I could not, plus the strain on my shoulders and arms with trying to quilt on my DMS meant I really avoided anything big unless I planned on paying a long armer. But I did rent time on one at the LQS and loved it! Just not convenient to do it that way-too far from home to work for a period and then stop and return later. So I dreamt of owning one, played on all of them at big shows and thought it would never happen due to $$. When my Mom passed (she taught me to sew)she left some $$ and my DH insisted I buy something I wanted that my Mom would think was cool (she would go to those big shows with me!). I knew it was a long arm. I found a used Gammill via my LQS and since the previous owner had died, I also bought all her thread, rulers, pantos, books,etc. It was a true bargain.
Couple of ideas I'd suggest-while my machine has a 14' table, a 12' would have been plenty.
And as far as throat space--you want to actually try that--it will depend on your arm length and its silly to buy larger than you can COMFORTABLY use
Make sure you can adjust the height of the table/frame--hydraulics are nice but expensive
Consider used--get the word out to LQS, dealers, at guilds that you are looking. That's how my Gammill fell into my lap! Watch ads in "penny savers", etc including auctions.
Don't spend a fortune assuming you can quilt for others and make a fortune--you may--but not right away and if there are many long armers in your area, it will make it very competitive--and many quilters seem to be buying sit-down set-ups for their own use.
After you buy one, do spend the $$ to go to a large show to take some classes, also check out machine quilting guilds, groups, etc in your area as they usually are very helpful.
Good luck! I love long arm quilting.