OK. You have to start somewhere. But you are not ready to consider steppers yet. How do you intend to MOUNT them to your mill. I don't know of anybody making conversion kits for that mill, so are you ready to fabricate mounts yourself?
Back to the Keling kit: These "kits" cost almost as much as the individual components. They are a good deal pricewise, but not performance wise--As configured. But, you can mix and match as needed and upgrade the price slightly.
Keling website:
http://kelinginc.net/
In order to get the full POWER out of the 425 oz (4V * 20 = 80 V) steppers--you would have to upgrade to the $190 72 volt power supply. But THAT would exceed the driver voltage. The 48 V Power supply would work with the drivers.
You would actually get more power by substituting the 282 Oz steppers which run quite well on 24 volts.
But you haven't determined the force requirement for the mill yet. No sense making mounts for Nema 23 size motors if you actually need Nema 34 size steppers.
CR.