You can probably emboss cardboard with a plastic or wood former, which any low-end CNC machine should be able to produce. But cutting dies are different. For those, you need to be able to cut steel fairly precisely, which isn't something that the low-end arduino-powered routers you're looking at can do. But look at the Taig and Sherline mills on my site; if the projects you have in mind will fit within their envelopes, then it should be possible. It's advisable to cut the steel in an annealed (soft) state, and then heat-treat it for hardness afterwards.
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