That's a very good start then. Is the mobo ATX or MATX?
Only need a chip, hdd, BR drive, tv cards and a case (with PSU).
If you can get some of those components 2nd hand then you'd almost certainly do it within budget.
gt
EDIT: The Zotac's probably pretty good - i've used their older Series 1 ION based MAG system in the past. Probably the only limitation might be that BBC iplayer's hardware acceleration doesn't seem to function (it did used to for a while?!?) at the moment so some SD and most HD stuff may stutter. This may of course be rectified by the BBC in the future but i've not seen any mention of it on the site.