I snagged a BEAUTIFUL Japanese skeleton keyboard for $35 back this past Autumn and extra $10 for shipping, and it is super awesome having a less wieldy keyboard compared to NA/PAL ones. Layout is mostly familiar and there are a few different keys but for the most part it is a slimmed down QWERTY board sans keypad, but absolutely awesome.
I did grab a matching clear Japanese mouse to go with my skeleton smoke grey console, but that cost like $63 total shipping included. Mouse is nice to have to have it for FPS games but as it is an aincent ball mouse it is not as ideal as laser ones. Also, Japanese mice are like 10% smaller than the ones from the other regions and it lacks the extra buttons those versions have as well.
The Brook Wingman SD controller adapter actually supports USB keyboards, not sure if they ever supported modern mice too but they at least did from the beginning support keyboards. So I mean you can use any modern mechanical keyboard on DC, but this one would be mental to own!
https://higround.co/products/dreamcast-keyboard