I use my 15-91 for FMQing and it does a beautiful job! I think you will be pleased. I FMQ with feed dogs down, but I found that it works just as well for me if I leave the feed dogs engaged and set the stitch length to 0. Either way, it does a great job. I purchased my low shank walking foot and a couple of different darning feet from Sew Classic that Cari mentioned above.
Just a word to the wise - if you haven't used the thumb screw to move the feed dogs in a while, you may find that it is stuck. Mine was, but I just put some sewing machine oil on it and had to let it soak for a couple of days before it loosened up.
Let us know how you like it!