as a former corvette guy and now a mustang guy i think sometimes people get a little wierd and carried away with this i have the only one made with Gray ashtray cover and A/C, realisitically all Saleen's built in the last few years are the same car. you can always come up with some weird combination.
from a historical perspective what's a rare car, anything under 100 meaning entire production, less than 100 exapmles of any variation. not 100 per year of the same car for 10 years.
when things get to the thousands, it needs to have a great pedigree, like a Ferrari. is a 308 Ferrari, rare, nope. is a front engine Ferrari Daytona Spyder yes, 427 Cobra, 250 made with an amazing racing pedigree.
needs to be VERY VERY unique, if we look at cars that have truly gone in up in value they were VERY limited production.
so in Mustang world they are rare, in collectors car world, not so much. that is my distinction. now 30 years from now, if you have a extremely preserved 'rare' example with less than 10,000 miles maybe you will have somthing. i.e last S351 built, or SA-10, but if you have a run of the mill 2000 S281 SC, i just don't see it.
and an even simpler test, if you can pop up the internet and find 25 of basically the same car for sale at any given time, i say it is not rare.
not to be an a$$ either, just my take.