My comment uncalled for. I apologize.
Though I have replaced every part in e36 cooling systems at least a couple of times, so far, all I have had to do so far at the front end of the two e46 engines was belts and tensioners. It is remarkably tight quarters up there.
I just walked out to the garage and examined the thermostat housings on the 325 and the 330, and I will concede that if one chooses to replace only the thermostat, that looks like it can be done without removing much else on the 325, just the air box and a few odd wires and the hoses.
This car has two thermostats, one is permanently fastened inside the thermostat housing that bolts to the front of the engine. The other is inside the expansion tank. My experience with plastic German radiators is that they crumble and fail after 8-10 years. My experience with German expansion tanks is that they crack and fail after 10-12 years. Rubber hoses can fail any time after 8 years, particularly if they are in a spot that gets a lot of engine heat. All these things are a lot easier to deal with if you pull the whole system out at once, replace everything at once, and rest easy for the next decade knowing that you don’t have to worry about your cooling system.
This is experience from four of the six cars that I have owned since 1990. The other two - my two '04s, have not yet had any major cooling system parts fail, but they both had O-rings on the water temperature sensor shrink and start dripping after 6 years.
BMW water pumps last about 150k miles, and I see his car has pretty low miles, so that may be a judgement call. That might be one of the “Is it my car or my wife’s car?” judgments. Or, it may come down to “I’m doing the radiator and heater hoses, my Saturday is shot, I am filthy and sweaty, I have antifreeze dripping into my eyes and armpits, I have two cut knuckles and a variety of muscle cramps from working in tight awkward spots. How soon do I want to do all this again to replace my water pump?” The alternative question is “How soon do I want to pay someone to do all this again to replace my water pump?”
BTW The water pump might come out with the radiator in place if the electric fan and shroud are removed, but it would be easier to take the fan and shroud out together with the radiator. The existence of a transmission cooler for an automatic transmission would be a factor in this decision. The water pump sealing surface is a lot easier to clean with the thermostat out of the way.