They're very simple pieces. BMW uses a metal design that lasts longer and is less prone to wear, although makes a lot of noise.
When I had my Skyline, the PS pump used aluminum veines that would expand with centrifugal force. They were known to wear our and straight up break in half, so you would end up with a pulsing PS. Not fun to drive.
I retrofitted carbon fiber veines from a Ingersol Rand impact gun for 12$, 10$ Oring from Hercules and never had the issue again.
Depending on which model E46 you have, you can switch pumps for better steering weight. The 99-00 use a LF20 pump, known to give a heavier steering with more feedback.
01-05 use a LF30 pump, known to give no feedback and make the steering feel very light. The pumps are identical and can be interchanged without modification. Same pulley is used as well. Look it up on E46F, almost everyone swaps to the LF20 when theirs dies.
