There are two main sources of info for the C5 (indeed, all Citroens) Firstly, I recommend you sign up with pr-net, which is Citroens own dealership system- you'll find exploded diagrams of all the various bits of your car, plus all the part numbers:
http://www.citroen-pr.net/spot/accueil.asp?lng =LEN
it's a bit of a weird site to use, but then it is a Citroen site....when you sign up just make up a pretend company name.
Secondly, sign up on:
http://www.frenchcarforum.co.uk/forum/
Now the C5 is coming into DIY ownership you'll be able to get info from people who have already done what you may want to do.