I replaced the water pump on my 323ci quite a while ago. It was relatively easy.
There isn't a great deal of clearance due to the fan, but manageable. I'm not sure how much clearance there is with the four door though..... at least you've got a manual trans so that should help.
There's only something like four 10mm bolts that hold it on, so once you get the belt off it's a breeze. O rings are used to seal it so there's no gasket scraping or anything.
GC Coilovers|GC Sways|SS S-Pipes|SS X-Pipe|SS Race Muffler|19" BBS CH's|Subframe Reinforcement