You might be better off trying to do a 'package deal' with a service provider. I got a deal last year at the Sou boat show, Ł1250 got me the phone, data kit +cable, installation software, a years subscription to a service provider (not minutes on the phone, but email compression software and a 'web diary')and more importantly ~ for me as a bit of a technophobe~ support over the phone or via email.
When I researched buying second hand phones there seemed to be potential issues with the phone software, apparently there are some very old phones out there. Initially I thought I'd buy a phone from ex-forces personnel as they sell them off (sometimes with remaining minutes) after returning from the Sandpit but I was told that buying a phone and selecting the best ISP is a bit of a minefield, even for those in the know. What seems like a good deal may not be.