Spec changes - basics - advice

With the prospect of diesels coming under fire in future and the possibility of a scrappage scheme on the horizon I am toying with the idea of changing my present 02 BMW 320D for a used petrol VRS hatch. I suspect that the latter will be less economical.  I would like to gen myself up on what main spec changes there have been within say the Mk II (= 05 - 13?) range that could influence my decision - things to "look for" (and things to avoid!).  Has the 2 L engine remained the same throughout?  Did it continue into the Mk III range, or do (early) Mk III's offer any significant improvements?  You get the drift I hope. Skoda's are new territory for me. Tks