No, the C6 rides well but its firmer than Xm and Xantias too but may be a little softer than a normal c5, it also seems short on travel which can make it seem a little bouncy and under-damped at the rear on undulating roads. In fact i think the new C5s ride better than the C6, though nothing has been as good as the Xm, a lot of people will say the Xm was never as good as the CX and that not as good as the DS. We have a hydraulic traction which is very soft, rear only on the tractions, but it handles a bit like a big mini, so direct its amasing, mind you need a few people to turn the wheel when parking!. However in my opinion the Xm is the perfect balance, as they are soft (can be made softer using various ways if you so desire, ours have a tiny bit more gas in to make them a little softer) but they have the hydractive which controls body control and long suspension travel means they rarely reach their limits. Still, the C6 is one of my favourite cars at the moment.
The 1.6HDi you have found is not a S2, for some reason the facelift C5s (from late 2004 onwards) were not classed as the C52, its still a S1 but a facelift, so i just call them facelifts, which is what my parents have. I have a late s1 pre-facelift, which has a lot of the added improvements to it (firmer suspension with tighter drive, slightly longer suspension travel, bigger wheels etc) which were brought out for the facelift. The new 'Germanic' C5 is the C5 S2, in fact.
76K miles on what reg? SX Spec? Service history? How much is it advertised for? Depending on that, condition and the work needed could be anywhere between £600-900+. Cheaper the better obviously

Can always start low and see where you get though, just give a list of what you think it needs doing when you make your offer.
They are great cars though, i can imagine you might be quite taken with a xantia, once they are working right, with their suspension etc they are superb.
Old fluid should not cause a brake judder, when you say its a slow judder, how do you mean?
Well my dad used to be a Citroen specialist, so sphere recharging was one of his main jobs. We still have all of the gear and still do our own Xms and a few others for old customers. You get a special tool which is used to recharge, there are 2 or more types of these tools and i forget what the name of ours is (usless i know). You also need the relevant nipple fixings to allow you to charge these, again there are various types. Ours have a big metal bolt which stays in the sphere and a screw on cap which is where the recharger goes in. This is very handy as it means spheres can be recharged after without removing them from the car.
Now, in order to recharge a sphere there must be a minimum of 15bar remaining in the sphere (some people use less than this but its not really good practice), if there is less there is a chance the diaphram inside the sphere could have made contact with the top of the inside of the sphere and has became damaged, which could just give up when more pressure is added.
So you also would need a sphere pressure tester, we have one made from an old GS rear suspnsion leg and a jack, though usually you can work out how much pressure is left in the sphere from the gauge on the regassing equipment, as it pauses slightly when it reaches the pressure in the sphere. Which is how we regass our own on the car. But with unknown spheres removal and testing is a must.
Now, if you can find any, some cars do have them fitted from new, if you get a sphere and it has 3 dimples in the top these are the prefferred shperes as they hold their gas longer. They have a 3 layer diaphram (much like the C5 spheres, which cannot be regassed by the way) and therefor lose gas much slower.
We used to recommend to customers to have their spheres regassed or replaced every 30K miles or 3 years, though ive heard of many xantias going for way more than this and still riding well, presumably these have the 3 layer spheres fitted.
So in short, unless you know someone with rechargind equipment or can get hold of your own cheaply then its worth buying new. Though id strongly recommedn genuine spheres, its all we ever use as we have had problems in the past with cheaper spheres which customer asked to be fitted. Its much easier if you dont have this recharging gear to remove the old ones (sphere removal tool might be needed though) and screw on new ones.
Sounds like you have fouhnd a decent xantia, you better be quick in case it sells! Though cars seem to be selling very slowly at the moment, they always do around christmas. I wish i had got my 206 sold by now.
My posts just seem to go on and on! I must try harder to write less, sorry if your bored

Chris.