Personally, I would treat yourself (or get someone to treat you!) to the Chambers Dictionary and Thesaurus on CD-ROM. This has the full text of the dictionary and thesaurus and is especially useful for 'full text' searches where one can enter a word and get every occurrence of that word in the dictionary - eg if one was to enter 'goose' one would get 'anserine', 'bargoose', 'barnacle' 'bernicle-goose', 'bird' 'boo' etc (really good this for general knowledge crosswords).
As far as paper editions go, then I agree the latest edition you can get or afford would be best, though you can often get an earlier edition cheaper in some bookshops. If you have a number of bookshops where you live, I would not expect you to have to pay more than �15 for the eighth edition, which I think is the 2003 edition referred to above.