Hmmm... they way I'd do it is set switches to be turned on at each key level (where you would be able to equip the armour or weapon). Then you can buy those items, or the chests with the items in would be able to open:
To use the switches you'd just have the events (eg shop and chest) with Conditional Branches:
If Hero [Zack] is Level 25
<>ENTER SHOP CODE/CHEST CODE HERE
<>
:Else
<>Message: Ha! Loser. You ain't got enough xp, n00b!
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