Trying it out is a great idea!
In the meantime, why not visit a sew center site that sells the Juki with a quilt frame to see how it lines up price wise.
She is upgrading to an eighteen inch mid-arm quilting machine, I suspect because she found herself limited by the size of the quilting area ... the Juki is barely bigger than your portable machine in the throat area.
Have you ever done any quilting before? You might want to practice with a pencil and paper, doing some quilting designs with those tools before trying out her set up ... it will give you an idea of the limitations that the seller ran into.
The last time I priced the Jewell, it was about $7,000 with a frame ... keep that in mind when you are quilting with this configuration ... when you start out quilting, you have about six inches of quilt you can quilt on. By the time you get to the end, you might find out that you only have two inches to work in ... it takes some pretty good understanding of starting and stopping the quilting design to make it all work out, but it is do-able.
I have a Grace Pinnacle frame with a Babylock Quilter Pro on it. While, sometimes, I feel frustrated with the limitations, they are not insurmountable, and the more I practice, the more I can do. <wave>