I build CNC plasma cutting machines (and repair them) as part of my regular business and the difference between what we build and what you need to reliably route wood is immense! We don't use stepper motors because they don't have the response time or speed capabitities plasma and oxy-fuel needs. BUT, we don't have to overcome latent resistive tool-force cause our tools don't touch the part. Any errors that are caused by the machine not getting to the next increment (due to friction or collisions) are caught and corrected for by the position encoders used to tell the servo system what to do next. The cheaper stepper systems assume that this will never happen and just counts step instructions given to the motor. Therefore, use the sharpest, highest horsepower router you can find to keep from bogging the steppers down. I prefer the little 4 horse air-routers made by ARO. They wiegh about one and a half pound and last forever, if oiled. When you are actually forcing a tool through material you are a lot closer to a CNC milling application and we don't mess with those. They are cheap, slow and powerful, just like you would want to route wood. And, wow, are they accurate!
Darn, almost forgot my point. The DIY retrofit kits and such that I have seen for mills are very much up to the task as long as you don't ask them to do it very fast. I helped an old friend setup a X-Y table to engrave plastic wiring-box seperators and it was a piece of cake but he thought it should only take ten seconds to do a 2 by 3 foot grid and, in fact, it takes about ten minutes. That, IMHO, is his bad. There is about 220 linear feet of routing involed.