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You should be able to find answers here -> https://charlesatan.medium.com/a-framework-to-determine-which-keycaps-fit-your-mechanical-keyboard-94f3f8b2dff5
 
The Z15 keyboards came with Kailh switches.

While it's a Kaih switch it should work with the traditional Cherry keycaps which are pretty much the industry standard.
 
As for the keycaps you would want it really depends on how long you want them to last without the letters wearing off. A good keycap will last a lifetime without the letters wearing off unless you are someone writing thousand page novels often. The pudding is just that the keycap has a white part around the base of the key that will glow with the lighting of the RGB lights. One major thing though to consider is what keycaps might you be looking to keep like the Macro keys and how they would look next to the ones you are looking to get.