If you are going to buy another Nokia N900 don't spend more than £100 or equivalent. Why not answer the following question? What do you want to do with your phone? (Edit: sorry, I see what you want to do with it).

m_dk29 mentioned Jolla. If I was you, I would wait for the new Jolla device or get yourself a cheap Nokia N9, I believe it can run the Jolla OS. At least then you are keeping up with the exciting developments, it is also open source and there is plenty of fun to be had with the new OS. The N900 hasn't died yet, but the future of Jolla I think is more exciting. (imo), only time will tell. Maybe Jolla will be ported to the N900, who knows?