2.5mm / 2.75 mm needles to give you the fabric drape you like; as I said, you may need to go up a size for stranded colour work. Work on circulars or DPN’s as per your preference.
4 ply / fingering / sock yarn in 3 different colours (see also suggestions).Overall totals will always depend on the size of the sock, but for a size 42 sock with 72 sts I used around 35 gr in each colours