The situation is different for Pentax than, say, a gold plated Rectaflex or some highly collectible Leica, say. Ordinary use doesn't harm the value, but on the other hand these cameras are common enough that having a few just sit on a shelf isn't doing much harm either.
OTH, in order to keep the mechanicals in running order, one ought to at least excercise the equipment a couple of times a year, even without film.
So: since most 'regular' Pentaxes are neither rare nor especially valuable, what one collects depends on what one is interested in collecting
Sounds like the OP is about looking at significant models, which the AOHC list is a good one for. Other possibilities: maybe collect the entire first set of K mount models, say. Or just models that look good and seem like fun things to display. Or, maybe, duplicate the models that we have documentation that the Beatles used
Here would be a nifty start: