I don't have time right now to read all the other responses, so hope I am not repeating.
First off, if you loved being able to move the machine, my advice is to get that kind of setup rather than a sitdown machine. If it's fun the first time you move the machine, it's going to continue to be fun for a very long time. If the sitdown machine is simply easier than stuffing a quilt down the throat of your domestic machine, it will be nicer than what you have but will not be that much ***fun***. Quilters who love the sitdown machine tend to not have fun moving the machine; for them, the sitdown machine is a good choice.
I have seen very good comments about the Block Rockit on other groups (I think Yahoo's homequiltingsystems and one or two others).
I would *not* recommend getting the Mega Quilter setup if it has a 9" throat. You will very soon (say, after two or three quilts) become frustrated with the harp limitation. It's worth investing more up front in something that will satisfy you for many years. For that you really need a larger harp space.
My Voyager 17/Hinterberg stretch frame setup runs $3,000 or less used. You might want to check the internet listings for used machines, Craigslist, your local quilt guild ads, etc. until you find one. I got mine from a quilt guild member who was upgrading.
If your budget could stand it, you might also want to look at an Innova. Their budget version runs about $8,000 and would retain very good resale value if you ever want to upgrade.
I have my quilt frame setup in our large windowless "mechanical room" that doubles as a storage area. I actually enjoy it in there. I take a glass of wine down there, turn on the radio and the bright lights (very cheerful), and quilt to my heart's content.