Funny coincidence, but I finished doing this on my 230SL just yesterday. Unfortunately, the 230SL and 280SL idler arm bushing setups are very different. In case someone is wondering, here's a thread for the 230SL:
http://sl113.org/forums/index.php?topic=3747 (look for the excellent post by Dr. Benz). I plan to do an additional write-up to describe my own experience doing this.
The part number for the 280SL rebuild kit is showing up as
1264600819, which is a much later number, so you might want to check with the Classic Center before ordering it.
The part number for the 230SL and early 250SL rebuild kit is
1114600050, and a the cap/upper bushing is
1114630133 (not included in rebuild kit).