I've used a cheap and simple iron from Rapid for ages (**broken link removed** model, I think) - my first tip lasted me the best part of ten years. Although it's great for light work on stripboard etc, you'll need something more beefy (50W or so) for larger components on multi-layer boards, especially now we're all going lead-free.
I was going to say you can't go far wrong with a Weller magnastat-type iron (there's very little to go wrong in there), but a quick survey on eBay showed that they're pretty pricey, even second-hand. At work we use the Metcal (now Oki) irons, which are very good, but another step up in price again.
As tronitech hinted, it would be prudent to try and get a tool that is (and will remain) well supported with spare tips.
I'm not convinced how much of a bonus an adjustable-temperature iron is, but my advice would be definitely to go for a low-voltage (not mains) iron with some kind of thermostatic control.