a "classic" car just seems to be an older car than would be seriously used as a daily driver, say 20+ years old but not old enough to be called a "vintage"
my alfasud could be called a classic, 26 years old, not many about and has a reasonable amount of "likeability" to it
engine size a factor? not really. mine is the 1.5ti arguably the best of the suds but is it any more of a classic than a 4 door 1.2? probably less so as the there are more ti's about than the rarer "lesser" models.
looking through practical classics magazine there is all manor of different models that at the time were considered "family cars"
original 70's cavalier (the rwd one, not the newer fwd)
even montegos & citroen dyanes
now all these cars were made in terrific numbers but have gone on to be classics.
so, a classic car, what is it
just a car that has survived the abuse of daily life and taken on to a cosseted existence. if a 156 reaches 20+ years old and still going it will become a "classic" by definition, automatically
and if you do
want to keep your 1.8ts to become that future classic then go for it, the ones that people will aim to preserve will be the V6 cars, the GTA in particular so your 1.8 will become the rare model that everyone forgot, cult status city!