Not a lot of differences between the years in reality, the changes are relatively subtle. The power keeps going up, but if you're not power mad then the earlier 276bhp model should be sufficient. Newest model has 309bhp, but are a lot more expensive at the mo.
There aren't many problems with the Z reliability wise. Major problems are rare. Main prob is clicky rear axle, but you can check for this on a test drive. This is usually sorted in the warrantly period anyway. Also subtle differences between UK models and imports, the main one being jap cars often are automatic, and UK cars are manuals.
Just as an example, there is a zed on here for £9995 at the moment.