This debate goes on on every car club forum in existence and never gets answered.
Does an MG TC owner think that an MG TF is a classic... No way!
Does a DB6 owner think that a DB7 is a classic... What???
Cars slowly slide into Classicness. They become accepted. It's not a defined or comprehensible thing. All cars become classics slowly. Is an Austin Allegro a classic? It probably is now, but not a great car I have to say. I do though admire anyone who can keep the tin worm away from one.
I would say that a Z3 is best defined as a junior classic or a classic in waiting. The Z4 is still a tad to modern, but it will slowly once again slide into classicness. They all do. Especially as they become rarer and a more modern look evolves.
It's a bit like trying to define what old age is. Some people are old at thirty others live there lives with a youthful outlook and a spring in there step and and never appear to succumb to age. Timeless.
All modern concept style cars appear to the eye as far to progressive at the time, but looking back in fifty years they will be very sort after. Note that convertible cars are always the more sort after over there roofed brethren. The thought process is that you'll only take the Classic out on sunny days so you'd like a soft top to enjoy them.
Can you actually imagine that when this car was new they thought about it being a classic? It was so cutting edge that you'd cut yourself. I can imagine people saying "Don't buy that as it'll date and be soo yesterday!" It was thought of as modern and stylish. Designed to the max. Not as a thing that will slowly drift into being an icon of a classic.
Relax enjoy your car and Classicness will one day be upon you. It will sneak up and get you in a slowly sort of way. Old age will also arrive if you're lucky so prepare and enjoy your car whilst you are able. Thats the way life is!