Do you have a Craigslist in your area? They often have good machines. Mine is populated with Kenmores right now.
I will answer your question on hearsay, not personal experience. I've heard that the add on zigzaggers aren't as reliable as the buttonholers. They do a screwy stitch once in a while and that's not desirable in satin stitching. I'd look all for a metal zigzag machine from the 60s or 70s. ebay should have something, too. My ebay machines have arrived in good condition, though one case didn't. Do you have a Goodwill or Salvation Army. SA here has good machines quite often. A sewing shop is going to charge too much (IMO) because they 'serviced' the machine.