The steering racks on the C5 X7 are a case of
when not
if. They don't favour 1.6Hdi or 2.0Hdi (or any other variant !), nor steel sprung or hydropneumatic - they're all prone to it !! As so many have been replaced already it is entirely possible that they've already been fitted with modified or improved versions i.e. not Citroen originals, but ones that steering specialists have reconditioned or refurbished. It is possible to check for pinon seal leaks with nothing more complicated than a torch

You need to shine it in through the driver's side wheel and aim for where the steering column enters the steering rack and areas below that. I can take a photo and annotate it to show you where to look if you want. You're looking for evidence of LDS fluid leaking. When they leak the fluid will run down onto the suspension components below the rack on the drivers side - so if there's any sign of dampness or LDS fluid in that area you've got a problem. Another way to check for a leaking rack on your own vehicle is to keep a very close eye on the LDS fluid level in the reservoir - that's very easy to do on a steel sprung model as the reservoir is quite small and the level is externally visible, but not so easy on an HPN model
Another point I forgot to mention - check both front windows go up and down without issue or horrible noises. The winder cables were prone (nay - guaranteed

) to failure.