No a Mustang II is not a rare car & it's not a classic by any stretch of the imagination. It is true that you don't see them around anymore, but you don't see Chevy Vega's around either. The best you could hope for in a Mustang II is the king cobra stripe package with a 302 V8 and T-Tops, and that's it. By the way Mustang II never came with a 289 engine as another respondent to your question stated.
No, the mustang 2's are the black sheep of the brand, they were good enough to keep the brand going, but basically being pinto clones they are not very well liked by most mustang fans/owners (plus they were pretty cheaply made)
I don't know if they'll ever be classics, (sorta doubt it)
The Problem w/ 70's stangs is the HP output was crushed by enviornmental constraints during the decade. Starting around 71 the HP went down every year, for example a 72 Mach 1 w/ a 351C only put out about 170HP (275 HP @ best)
However early 70's stangs are just starting to become collectable/desirable, whereas a few years ago they weren't. So really only time will tell if they'll ever be worth it to anyone. But as for the immediate future, don't count on it.
If you're looking at it to buy I would either go 73- or 79+ if you want anything of value
a 33 year old car is certainly a classic vehicle whether any one believes it or not. try to find a 2 door ghia or a 3 door mach 1. probably most have been sent to the crusher. it does not have to be a muscle car to bring big bucks now days. a mustang II with a hot 302 makes a mean little stop light rod. the main thing that hurt [in some peoples minds] the mustang II is that you cant put a bigger motor or tranny in the car because of the way it is designed. see if anyone can find out how many cobra fastbacks are around. check Craig's list in St. Charles.
They are rare because nobody saved them. They went unloved even though they were a top seller in the early 70's ..they were the littlest Mustang and even offered a 302 v8 from 75 to 78 and Cobra packages too..
and they do have a fan base