Go to Cases website and join their official collectors club
http://www.wrcase.com/ that would be the best place to start.
As for what makes a Case knife collectable there are a few things to factor in: How many pieces in a run, Handle material, Knife style, if its a limited run. All these can be factored in together too. Case makes a line of knives called Case Select, these knives are limited every year to a certain number of pieces, and they are different from their other knives because of this and because of the combination of handle patterns and materials. Case Select knives are even limited as to where you can buy them.
Another thing with Case is the Case Long tail C stamp that is put on a limited number of knives making them more collectable.
I know if you join Cases Collectors Club you will have more information on these things, another place to learn is at knifeforums.com they have a large group of Case knife and slip joint fans.
I dont know about touring the Case factory but I have been there and they do have a museum and store, although the biggest part of it is Zippo products, I think a lot of Case memorabilia was stolen buy a former CEO or something when he left to start his own knife store.
Hope this helps.