You should replace both left and right shocks.
Put the reg into Carparts4less and it'll bring up shockers. Phone them to confirm it's for the SI suspension.
Sachs Shocks for mine...
Front Left £107.61
Front Right £107.61
Rear £68.18 each
Front Springs £55.16 each
Rear Springs £91.32 each
Both front wishbones and lollipops.... £89
SUSPENSION TRACK CONTROL ARMS WISHBONE LOWER
FRONT FULL KIT FOR BMW 3 SERIES E46 & BMW Z4
INCLUDING BALL JOINT, RUBBER MOUNTING, MOUNTING BUSH, ANTI ROLL BAR LINKS AND TRACK ROLL ENDS
https://www.ebay.co.uk/itm/Wishbone...162386&hash=item4b37d9c261:g:8zkAAOSwstJZQQ-t
Rocker Cover Gasket £12.93
https://www.ebay.co.uk/itm/Rocker-C...616718&hash=item4d5f2bd538:g:ikcAAOSwZHVbvQna