One of these:
https://www.wotol.com/1-index-745-ve...prod_id/758776 ? That's a full-size knee mill that resembles a Bridgeport. It would work better if you replaced the lead screws with ball screws, but it would probably work to some extent with the screws it's got. It will cut steel better than any home-built router, not to mention one made out of 3D printed (plastic) parts. The spindle speed seems slow, like it only has the low range and not the high - most mills like that will go to 5000 rpm or so. But if you're mainly cutting steel with large cutters, it should work okay, and if you need to cut other materials or use small endmills, you can fit it with an auxiliary spindle or a spindle speeder. You can either use steppers (go for at least NEMA 34) or servos to run it. Steppers are easier to set up and a bit cheaper, but servos are stronger and less likely to lose position. If this is the only mill you've got, keep it in working order so you can make all the motor mounting plates etc. you'll need for the conversion, and wait to tear it apart until all those things are complete. Work out the plan in detail first, and use the intervening time to research control systems - give that a month or so before spending any money on any of that.