You can FMQ on a frame without a stitch regulator. Angela Waters did it for many years! I'm not that good, but I found it to be a lot of fun. My recommendation, though, is to do a ***lot*** more online research before making a decision.
Domestic machines such as your Juki have only 9" of space under the arm (harp area). The takeup bar on a frame "takes up" a portion of that space. As you roll your quilt, you have less and less maneuverable area under the arm such that, with a twin size quilt, you can easily end up with just 2" of space to FMQ towards the end. This drastically limits the kind of FMQ patterns you can do.
A Voyager 17 with Hinterberg frame (my setup) can typically be found used (they are not made anymore) for less than $3,000 (sometimes $2,000) with or without stitch regulator. This is a genuine midarm (the 17" under the arm of the machine) which allows you a lot more flexibility and variety in what you can do.
Investing $600 in a frame can be fine as long as you understand the limitations of using a domestic machine on it. Most people who enjoy frame quilting outgrow the 9" space very quickly and want to move up in size. The $600 frame that works with your domestic machine will probably not work with a midarm machine as you need a larger carriage. Also, Grace does not have the best reputation for midarm and longarm machines; they tend to be a bit on the weak side. It is hard to sell a used domestic machine frame, so you may not get much of your money back if you decide you want to upgrade.
It takes some research to figure out what both fits your budget and your needs. A good place to do that kind of research is on the homequiltingsystems group at groups.yahoo.com . That particular group has many members, and their setups range from the one you are describing all the way up into professional setups. Might help you make an informed decision.