The pc meets all the recommended requirements for Counter-Strike: Source, except it has no Operating System according to the listing.
1000W power supply? Huge overkill, and looks out of place. Says it can be overclocked - does it have an upgraded heatsink in it? I would double check those issues first.
150 pounds for old hardware like this is not a great deal, but it may be sufficient if you are looking for a machine that will suffice for a couple of years.