Rhein 2217's are amazing!....I have been running two of them for several years with great success. In regards to the FX5 on Craigslist, bring a container and run the filter for at least 15 mins! Make sure it works with proper flow rate. I would also research online what common problems you need to watch out for with those filters and make sure the one your are buying isn't showing any signs of those problems. It's so much better to be thorough then regretful!!!
Really, besides the seals and those top screw things, the only things you typically have to worry about on a used FX5 are either the impeller (simple cheap-ish fix) or the motor assembly ($90 fix). The entire motor pops out with 4 screws, pretty simple. So, if you can get a used one for $100 with all other parts, worst case scenario you can still basically get a good as new FX5 for $190. I would hit the hardware store and get a few feet of 1" ID hosing to test it. Remember it turns off after 2 minutes for 2 minutes...