On some vehicles, particularly front wheel drive ones, the water pump is in the same, difficult to get at area, so most garages recommend changing it at the same time as the cambelt.
This logic does not apply to TDi 200 and 300. The water pump is straightforward to get at, without touching the cambelt timing chest.
Just a couple of tips for you:
1. Slacken the three pulley bolts before you remove the belt, makes it a bit easier;
2. Make sure you have plenty of new coolant in stock as you will lose some when removing the pump, ensure you have a large enough collecting vessel (10 litres);
3. Ensure you clean off all traces of the old gasket, before installing the new pump;
4. Put a
thin sliver of Hylomar Blue sealant on both sides of the new gasket;
5. Install all the pump retaining bolts at the same time, but snug them up in a diagonal fashion to ensure the pump settles correctly. There are about six bolts from memory, so do one at 9 'o clock, then one at 3'o clock, one at 6'o clock and one at 12.
In other words don't install one bolt, bang it up tight before moving onto the one next to it.
5. Refill the coolant circuit via the thermostat housing, when it's full, replace the plug and let the engine run for a bit. Then check the level in the header tank. You make find the level drops over the next few days, this is due to air locks working their way out. NB. monitor this level carefully until it has fully settled, topping up as necessary.
Honestly replacing the water pump
IS an easy job.
If you haven't considered it, replacing the 'P' gasket would be a good thing to do at the same time, if it doesn't phase you too much.
PEB500090G | Water pump and gasket - OE supplier - 300TDI
ERR4686B | 300TDI Thermostat Housing Plug - Brass
Hope that helps.
Dave