Go for something like this. These are very quick, good spec machines easily capable of what you want it for. You need to buy a screen on top but still well under £500. Refurbished' in PC World terms means there was a fault on a brand new machine customer sent it back and they fixed it, now sold as refurbished but effectively brand new.
http://www.pcworld.co.uk/gbuk/packard-bell-imedia-a3622-uk-refurbished-desktop-pc-06200185-pdt.html
http://www.pcworld.co.uk/gbuk/packard-bell-ixtreme-i5622uk-refurbished-desktop-pc-06200160-pdt.html