Looking for something to keep for years and years, enjoy during warm weekends and hope that it is collectible.
Choices are:
Porsche Boxster
Audi TT Quattro Roadster
Chrysler Crossfire (Coupe, Roadster, or SRT-6 Coupe)
2004-2006 Pontiac GTO
Dodge Ram SRT-10
Chevrolet SSR
2001-2003 Corvette Z06
Or should I buy an already classic car?
I don’t think any of the cars you listed will be a classic during your lifetime.
I’ll run thru your list real quick though and give you an opinion
The SRT-10 Ram is a joke… slow, very slow, poorly made… the newer ram is really nothing to lust for, ever. I drove one with 30K miles for a while to pick up a friends Kawasaki, and it was the worst truck I’d driven in a while. The "HEMI" was a joke, the doors buffeted in the wind, etc…
The SSR is loved by some, I guess they enjoy the fact that it closely resembles a clown…
The Crossfire SRT-6 is an okay little car, but it all ways comes in last when its compared to the other roadsters in that market. Its a worked-over Mercedes SLK underneath, nothing new.
The GTO, hm… never will be a classic, I’ll tell you why.. there are hundreds of thousands of them in Australia and elsewhere with Vauxhaul badges on them. Not that its not a great car, but the new ones I doubt will go up in value for at least 50 years.
Boxster, hm. Slooow Porsche. Generally referred to as the girl Porsche. Easily out performed by many cars its compared to, like the BMW M3, S2000, etc. It is a Porsche yes, but again I don’t see it going up in value soon.
The Audi TT Roadster is a great car, esp if you looking at the V6 with the DSG6 gearbox. A real sports car. The value of it is questionable though.
Ahhh Z06… no platform sharing, as is the case with all of the other choices other than the Boxster, its made with one purpose as well, and not a compromise like most of the other cars here. Your best bet a retaining value, and possibly increasing in the future. Being a Chevrolet it will probably plummet for a decade or so at least. Especially because its newer badder brother the C6 Z06 does everything it does better and has been recognized as a real threat to the supercar manufactures. However, it is the best performer you listed and the most respected across the board be it a foreign or local crowd, dragrace, circuit or cruise.
If you buy a nice one with low low miles and maybe a few rare options, like I think they were available one year with red interior, I would put my money on it for being the most valuable of your choices, and also the one to retain and increase in value the soonest.
Just my opinion. =)