I can not really help you with the installation; my Xantia was a petrol engine and yours is a HDI so it is very different. I installed an AP 800? on the C3, but the C3 is with electronic gas pedal, so installation is really only to click on the plug. Very easy and it still works very nice and stable
Guess someone here must have done it on a HDI and be able to give you a few tips and tricks.
Good luck / Mads