If its been serviced, cased with all the original accessories inc. both 5 shot mags and the single shot adaptor you'd be lucky to see one under £350...if any of the above are absent or its never been serviced then £250 would be the bottom end of the market.
As for shot count - I get 80 shots per 12g powerlet set at about 450fps obviously if it was set higher it would be less...
You can't really get larger cylinders without a lash-up of some kind although the standard accessories contain what you need for bulk filling the gun.
(Other than a CO2 supply & maybe an adaptor depending on your tank)