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Some good keyboards you have already
The 60% AEKII sounds an interesting project. You'd need an AEKII (obviously), a plate (unless you can cut down the AEK plate). A case (unless you want to cut down the AEK case, although that kind of shenanigans doesn't usually end well). Or perhaps you can find an existing case that would fit the plate and controller that you choose.
Finding a plate is probably the first step, and deciding whether you want to have a PCB or make it hand-wired. Probably.
What would I know - I've never built a keyboard