Those look like pretty small diameter ballscrews, my guess is they were used because the previous owner figured out how to fit the ballnut inside the existing leadscrew mount. You can order all new balls in case you lost any. Nema 23 motors would work just fine. I would gear them down 2:1 with a timing belt so you gain finer control of the table movement. 1 step of a 200 step motor with a 2:1 ratio and 0.200" lead ballscrew equals 0.0005".