Great choice. I'm no expert, but I do have a older '64 and a newer '06, so here's my $0.02: AFAIK, there really aren't any quality issues between new and old - are fine rifles and shoot great. My '06, though, has a rebounding hammer safety, whereas the safety on the '64 is a half-cock. The '06 has nice checkered walnut, whereas the '64 is plain. The action on the '06 was fairly stiff when new, but is smoothening out, but isn't near as buttery smooth as the '64, though I suspect the '64 was fairly stiff when new. Finally, the muzzle crown on the '64 is nice and recessed, whereas on the '06, the muzzle is flat, with an ever-so-slight bevel at the crown. Still, the '06 shoots great. Maybe I'm paranoid, but if I were to have some work done on it, I'd probably re-crown it - for nothing other than to help protect the crown a bit more.
The version that seems to be very desired, and from my experience, pretty rare, is the Mountie carbine. Even though I already have 2 39s (and several CZ452s), I'd buy a Mountie in a heartbeat if I came upon one.