Im about to make the plunge into a CNC conversion of my old Jet mill drill. It is essentially the same an RF-31 or a Grizzly 3358 (http://www.grizzly.com/products/G3358)...over 600 Lbs, round column, R8 spindle, 9x24 table.
Im looking at the Keling kit (http://www.kelinginc.net/ThreeXCNCPackage3.html) with 495 oz in. steppers, 4030 driver, 24v power supply.
I plan on using ball screws (7/8" x 0.200") and gear reduction (only if I need it for the torque).

Ive done lots of reading and searching here, and found all the sites (I think)and threads with these conversions.

From what Ive seen, I estimate these to be powerful enough for milling aluminum and steel on this machine, but just want some reassurance before dropping the money.
Rapid travel is nice, but with only 24" of table, I wont be going far.
Those motors seem so small, its hard to believe they will be strong enough.

Do you think that kit is a good selection for this application? Do I need better drivers or Power supply?

I have basically the same mill/drill from Busy Bee. When checking on the table feed motor from them it is listed at 200 oz. I have bought 300 oz steppers for mine and am going to gear it 2 to 1 to start with. I am hoping that I can do what you are doing before too long. My spindle motor is 2 hp and I don't see any problems unless I get carried away with to deep of cuts.
Keep us updated on your progress as I am interested.
John