I have rebuilt my P/S pump myself, before it was leaking a few drops a day.
To me the structure of MB's P/S pump is very simple, it's very tough inside. there isn't a thing that can be easily broken, after 220k Km, my P/S pump doesn't show any wear inside, but the front radial seal and rubber O rings are so bad that nearly every 400E I have seen leaked P/S fluid, also sometimes the hose (high pressure hose and reservior hose) connectors leak.
I took me a day to rebuild my P/S pump on my 400E, for none V8 W124s, it may be much easier.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.