What are you putting the encoder on? Spindle, axis drive, etc.?
If you're looking for an encoder for an axis drive, then the servo will come with one. Suggest buying a matched set of servo & drive and then you don't have to worry about incompatibility.
If you're looking for a spindle encoder, then find the drive (VFD?) you want, then you can figure out what encoder is appropriate based on output circuit/signal type, voltage, and encoder count (PPR). Note that not all VFD's can handle encoder inputs, and most of them can't handle very high signal frequencies.
If you're trying to re-use the old servos and put new encoders and drives in the machine I'm not sure how that would work out. I think servo encoders are generally in the multi-thousand PPR, not 600PPR.