this is a tutorial on how to use a conditional button device: https://www.youtube.com/watch?v=6G3nVBuwFXk&pp=ygUXVUVGTiBjb25kaXRpb25hbCBidXR0b24%3D
That is one method to use, and probably the most straightforward- to manage which damage amp they get though it kind of depends. Either way is going to be a little complicated for a beginner but you can either use Verse and set the damage amp to the magnitude you want on each purchase, or you can use switch devices to keep track of which level they are at.
As far as the vending machine goes, the item spawned event does in fact send an instigator and can connect to whatever system checks for what damage to give you. To display the damage amplifier on it, you’ll have to change the material- which there are tutorials for that on YT as well.