This is not a recommendation, just one data point from one user to get the ball rolling...

My gantry router uses the ubiquitous 3HP 24K RPM Huang Yuan (or is that Huan Yuang?) water-cooled spindle. Fine for cutting wood and plastic, and also done some fine engraving in brass without problems. I have also cut steel, generally profiling cuts in up to 3mm steel sheet, but the problem is minimum spindle speed. I seem to remember that this spindle is notionally rated down to 6K RPM but I found that I could stall the spindle at that speed if not very careful. I ended up running at about 7K minimum with 3mm carbide cutters and DOC set to not stall the thing. Mine is a fairly stiff steel-framed machine. My feeling is that you could do a fair bit of steel cutting but slowly! I later bought a CNC mill running the same cutters at about 4K and that works much better. In general, there is a limit to "one machine for everything". Probably why there are examples of people adding a high-speed spindle to a conventional mill for when those high speeds are needed.

I'm sure that others will have different views!