I recently did the same repair to our 300SE (126). I'm not sure how your pump is configured, but but here's my experience.
The one for our car came with fresh o-rings so gasket replacement isn't a issue. By the way get one from Fastlane, in addition to helping our hosts, it's OE and you don't have to worry about the core.
I found this job to be a real pain in the rear, had to remove numberous pieces that aren't defined in service cd. like: radiator (to access viscous clutch), all pulley's, belt tesioner, power steering pump. All this to get at the four bolts that hold this rascal on. Additionally my CD said to remove the AC compressor, I did but you don't have to do it.
That being said just budget a few more hours than you expect, take your time, and the job is not difficult. Also be sure and change the themostat while you're there. Hope this helps.
'91 300SE sold
'92 190E 2.3 sold
'06 Mini Cooper S
'08 Triumph Rocket III