As far as I know, there was the E3 model, then the Sport model and then the Vibe.
So I'm not sure what people mean by the classic version (except maybe to differentiate between the new Vibe and the Sport model, which I believe are still sold).
The main difference between the E3 and the Sport was that the toddler seat (when it is at the back) reclines slightly and also there were finger guards over the back wheels to prevent fingers being poked in the wheels. E3 model will be older and possibly more used than the Sport.
The Vibe was a completely new model. I don't know all the differences but I think one of the main ones was that you can fold the buggy with the toddler seat in the rear position (you have to take it off on the Sport model).
TBH, the reclining back seat on the Sport model is a waste of time once the child gets over about 12 months of age because once they are over a certain height it has to be in upright mode otherwise they bang their had on the horizontal bar that goes across the top of the buggy. So if you could get a cheap E3 that looks in good condition then I'm sure it would be fine).