I think they updated the Bonneville in 2006
with a slightly bigger engine/ more power, so it may be worth negotiating a bit because of that as I'm fairly sure the 2005 is a different version to the new model...
I'm just going to hunt a bit and see if I can find more info for ya!
EDIT: Here from Wikipedia for you:
The new Triumph Bonneville produced by Triumph Motorcycles Ltd was released in 2001, and has a 790 cc parallel-twin engine of the same configuration as the original "Bonnie" as they are collectively known. The T100 Bonneville received the 865cc motor when it debuted. All 2007+ Bonnevilles have the 865cc motor. 01-07/08 Bonneville engines use a carburettor (as opposed to fuel injection), chain-type final drive, and other features consistent with the original production Bonneville. Except in The United States, All models from 2008 will be fitted with Electronic Fuel Injection to comply with emission regulations, in the United States EFI will debut in 2009.
Yes the Euro emissions regulations seem to get more and more strict each year. They've killed off carbs now. I think the newer Bonnies with EFi have dummy "carbs" hiding the injectors - a nice touch really.
My mum has a 2007 Bonneville and is very happy with it.