I'm relatively certain that you'll be disappointed with a £200 ebay iMac. Just a guess; I'm not an expert on the ebay sweet spot, but it looks like you're mostly still bidding on old, slow CRT models in that range? Lesson #1: Macs have fucked-up resale values.
Since you probably already have a monitor, a USB keyboard and a mouse, I might suggest you look at a new Mac mini, which will handle itself ok for your everyday computing. Once in a while I wish I had more RAM, but otherwise I'm not unhappy with mine at all. It looks like they start around £360, and if you don't go too crazy with the video stuff, the cheap base model will be plenty at least for an introduction.
As for "getting into the scene", I believe you'll need to PM Canis and he'll come around with his special paddle. I'm told it only hurts the next morning.
For those prices I dont know what you'll be able to get. As far as new computers go I'd guess you'd be able to do it with the Mac Mini, but you'd have to supply a monitor and keyboard/mouse. The cheapest full mac system that would do that is either the iMac G5 or the iBook. You can do all iPod, office, and net applications on a 600MHz G3 with 512MB RAM, however video stuff depends on what quality of video production you're into. Apple suggests 1GHz G4 as a minimum for HD-quality video production in iMovie (a video application that ships on all macs).