i got the aeron remastered for 750€. its really good but i miss a headrest.
Get a gym membership instead
2ndhnd.com used to be a good site, but now that UK is no longer on the EU I guess you could face some tax if you import one from there. Btw, you can search on ebay or if there's somewhere in your country that sells them, usually when an office or bank close there's some stock that has good condition.
I have a Sedus se:do pro and is not as good as a HM or Steelcase but it does the job. Btw, the best way to know if a chair fits your body, is go to a place and try it.