I have a brother nouvelle pq 1500s. It is a great machine for Fmq. And piecing. Mine has a very nice straight stitch and is not at all picky about thread. You can sometimes find them new online For as little as $650 but they usually run $799 and up. A typical price for a used one is between $400 and $600. There is not much that can go wrong with them since they are a mechanical straight stitch only machine. For $550 I would expect it to have all the original accessories that came with it including the extension table, all the feet, the extra pin for the pin feed, the fabric separator etc. At that price it should also be in excellent working condition and I would also expect it to be cosmetically very good.
Rob