Changing the clutch
First of all, shop around for the clutch--the original Citroën ones are very expensive (exceeding 500 EUR), so I actually went to the shop and opened up a clutch kit. It turned out it had parts from Valeo and Sachs (I have been told some also have Luk parts). I looked up the suppliers, and found an XM kit from Sachs with a three year warranty. When I checked it out it turned out to be exactly the same kit down to the Valeo parts. The only difference was that it cost less than half the price.
The easiest way to change the clutch is actually to have the whole engine out.
While changing the clutch, it is very advisable to replace the crankshaft seal on the gearbox side preventively. At the same time, don't forget to check the shaft surface where it touches the seal. The cost of the seal is negligible compared to the cost (mostly in work involved) of the clutch change.