Just gotta chime in here:

I posted a DIY several months ago on rebuilding the w123 SLS valve here:

DIY: Rebuild the self leveling suspension SLS valve on w123 300TD wagon sagging rear

And, as of today I still have 2 full sets of o-rings available for $5 each. It's not as nice as DieselDiehard's facotry rebuild kits (no piston, springs or BB), but its about $100 cheaper.