I don't don't have experience of the Mk1 or Mk2, but had a mk3 golf driver & then a mk4 1.8t. My family still think it's funny that I call the mk3 a hippo - well, that's what it drove like. My 1.8t came alive after a remap and then I think I lost my senses with a k04 conversion, R32 everything, TT short rack, and it became a bit more of a fun car.
There seems to be a lot of ED30's up for sale at the minute - 5-dr's seem to have lower miles, weirdly.
I think they are already at 'modern classic status', all I will say is that I think if you look after them, they depreciate more slowly than most. Bought mine on 47k for £12.5k from VW in 2012, now on 104k and maybe still worth £7k - maybe a bit more with the options & history of the car. Still, as ever, it is whatever someone is willing to pay... I'd rather drive the thing than polish it / look at it in the garage...
I still think the Pirelli will be the classic of the mk5 range, as few know what it is!