Stiffy.......greetings from Brizzy

I replaced the water pump on my 1978 280CE w123 several years ago

I had to remove the harmonic balancer.....but it comes off quite easily.........you don't have to undo the crankshaft bolt.......which is just as well as it is at ~400Nm

There are 6 X allen head bolts that come out quite easily if you have the right tools...........buy a good set of allen sockets.......mine are Stahlwille and Gedore (both German) which I got from Trade Tools........looking at mine I think the bolts were 8mm, but you can check the size first with an ordinary allen key

I have a Haynes manual, though not great does cover the pump and balancer removal

Good Luck!

David