I think just about every mini bike brand is collectible in one way or another. I have found that HPE/Muskin Cats do tend to be very collectible, especially some of the more bigger models like the Dominator, Eliminator, Wildcat, Duster MX, 400X, etc. They are mainly more collectible because it is easier for all of the "grown-up" people to ride them compared to the smaller models.
If you could show us what kind of Cat you are looking at we might be able to give you a little more help in making a decision. When it comes to a restoration of Cats, having more parts when starting is better, because Cat parts tend to fetch high prices even when they are in rough condition and you might also have to spend years looking for a part if it is a very rare part.
I am personally a fan of HPE/Muskin Cats so they are definitely collectible to me!