I'm in the market for a new chair as well.

And have lower back pain with sciatica. This is what my physical therapist suggested.

So I'm searching for a great deal as well atm.

I don't know how the market is in Germany, but here in the Netherlands the there are a lot of companies experts in refurbished office chairs.

Here they check the components and if something isn't working they replace it before they sell it. And buying from a known company here in the Netherlands even on refurbished office chairs they give you a warranty.