I have a retired M-series Auto. They are very capable, comfortable machines. The downside is they use a Smartcard to record data, and the only way to access that is to use a special card reader (Mako Infineer DT3500 USB) which costs $59.
offers used machines with a warranty, and that might be a better bet for you. Also the newer generation Philips Respironics System One 60 series Auto machines are available new for $400 on Amazon and at other locations (I know, too much).
I bought a PRS1 Auto for $200, and a PRS1 Bipap Auto for $350 last year on Craigslist. Check your local Craigslist ads, and use SearchTempest in order to search for machines outside your local area but nearby. SearchTempest is a national search engine for Craigslist listings, and you can enter terms like Resmed S9 Autoset, or Remstar Auto, and get lots of results. If you are not a smoker, beware of buying from anyone with a smoker in the household. The odor is not easily removed. Also, lots of people have a ridiculous idea what these machines are worth...be ready to negotiate. Meet in a public place where you can plug the machine in to complete a transaction.
Get a new mask. Many of the online sellers offer a 30 day return if it doesn't work out, and using someone else's mask is just not a great idea. You seem to have enough knowledge to complete a good transaction. The deals you have found so far are not exceptionally good. Keep looking. Also, fill in your profile so we know of your general location, and we can try to alert you to any deals.