There's an o-ring between the water pump and the block. It's less than $2.50 from the dealer if you can't wait for FastLane. I used a dab of sealant to keep the o-ring in place while wiggling the water pump into place.

I dismissed the recommended Snap-on tool because I can get to the upper rear bolt with a 1/4" socket arrangement. However, I had to remove the dipstick to use the 1/4" socket arrangement and that allowed coolant to drain into the sump. If the Snap-On tool allows you to get to the rear upper bolt without removing the dipstick, then it's worth the money IMO.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD