being a newcomer i would probably go for a kit, like the cncfusion kit. I plan on converting my own x2 mill (when i get some money). but i engineered my own mounts, ballscrews, ect. it was fun and i am learning a lot from it.
yes ballscrews are a must! if u want any kind of accuracy that is. Id go for the preloaded ballnuts (a bit more expensive but worth the accuracy), they just put larger balls in the nut to remove some clearance.
i plan on going with steppers in the 250 oz in range, 425 is a bit overkill on a small mill like this, and speed/torque will be affected at higher speeds (a smaller motor will be faster, but not as strong, and vice versa). I might use a 425 on the z axis, just because of all the mass it has to move.