300SD upgrades?

Hello,

I have a 126 300SD and wanted to upgrade the rims. The car has the factory alloy 14' rims (old style). I like the rim style on the 300SDL series, however they are 15' rims. Can these be used on the 300SD? What tire size would one use? The original size is 195/70R14. Can 14' rims be purchased in the new style? Can anyone recommend a good source for new/used mercedes style rims? I live in California.

Also, I also wanted to replace/upgrade the OEM stereo speakers? Is it best just to stick with dealer replacements, or can aftermarket upgrades be installed. The OEM speakers appear to be about 6" x 3"

I have also heard about K&N Filters for mercedes, promising a performance boost. Has anyone had any experience with these filters on a diesel and are they really worth it?