Not much to buy: Eurocarparts or German,Swedish, & French will sell you the bits, both are online and have shops all over the place.
If fitting yourself don't forget to wind the caliper pistons back, you don't normally need the special tool to do it-most times you can do it by hand, or with a bit of help from a screwdriver.
Re Your Xantia. If yours is the facelift model that doesn't sink when park you need to make it go up and down a few times now and to the 'exercise' the suspension parts. The early models with Chevrons on the bonnet did it itself. Ive also got a BX and would advise you to join thebxclub.com as many of the members also own Xantias and there is a wealth of knowledge amongst them and all good blokes too! Good luck. Geoff