"Prepared-ness" is a weakly descriptive term, which makes it hard for me to parse out exactly what you mean. I'm going to interpret it as "anticipating future events and taking specific actions the player would not otherwise take so that he or she can meet those challenges".
I think that the big issue with preparation in games is that it requires operational knowledge of what will happen, which is trivially easy to gain by failing and then reloading, at which point any possible preparations have been revealed and will be undertaken as a matter of course (as opposed to contingency planning). With the complete knowledge already gathered, prep becomes indistinguishable from the thing for which you are preparing.
Giving this information in advance is tricky. If the danger is clearly and conspicuously presented, then prep is more or less a simple chore, not a choice. If the dangers are too vague, then prep is impossible. If I have to happen across some character or sign to know of the danger, or the danger is cryptically hinted at, then I have to take arbitrary actions just to have the opportunity to prepare.
Procedurally generating situations for which players can prepare sounds difficult to me. How does the game analyze what the player is "trying" to accomplish, if anything, by taking a given action? How many possibilities can the game throw at the player, and how clearly can patterns be discerned? How bad are the consequences for the player wrongly anticipating the future? Going with the bullet-scarcity example above, a panicky bullet-sprayer will find more spare ammo to replace what is used and never need to prepare. A bullet conserver, on the other hand, will have an inventory generally more filled with ammo and so will lose out on other items they can't carry as well as the plentiful bullets there would be if not for their own caution.
For me, the best mix of factors to model preparation in a game might be:
-Some degree of choice in what dangers I'll face
+I can go to Thief Hideout Mountain or Zombie Canyon
-Clear information, complete or otherwise, on what I might encounter
+I'm headed to Zombie Canyon, suggesting there might be zombies in my near future
-Successful preparation requires some thought on my part about what might work
+I have to draw a connection between some item, ability, or other game feature and the danger I expect so that I can plan what I think will be effective
+Preparation does not involve having everything laid out. To prepare for Zombie Canyon, I should not be required to speak with Zeke the Zombie Slayer and suffer through his associate's course on zombie-killing to prepare, learn how to prepare, or be able to prepare
-Preparatory steps which are used for more than just facing one obstacle and/or are only partially effective against that obstacle
+A healing potion can restore HP for my character or hurt zombies (an item like "Zombie Repellent" is too obvious and too specific to feel like a choice I can make)
+OR a healing potion can help against zombies in a way other than just hurting them (such as, slowing them down or making their attacks weaker)
-Forced choices between competing dangers and their preparatory steps
+Some constraint prevents me from prepping for all or a wide variety of events at once, meaning I must always make an explicit choice to be ready for one danger or another
-Significant, but not extreme, outcomes whether I've prepared well or not
+I miss out on something cool, or have a much more difficult time, if I have not prepared for Zombie Canyon before going there. The game should not end, nor become unwinnable, because I prepared badly
+Preparing well provides something cool, or makes clearing an area easier, but does not provide such an incredible benefit that it becomes de facto mandatory to prep
-A variety of well-defined situations (or combinations of situations) for which I can prepare