Well yes they should do, though GSF ones should easily last 3-4 years or 40K miles without losing too much comfort.
Have you looked on:
http://www.aepdirect.com/ for parts? They sell spheres etc. Usually give good service but ive no idea what their spheres are like. At least many people on here use GSF and they seem to be decent. Id buy them, GSF ones.
Well sometimes PUG drivers (no offence, im one too!) pinch the 2.0TCT from Activas and Xms to put in their 205s etc. So a shell is left, if you ever saw a shell available that would be ideal. But generally the TCT is a strong engine, its usually leaks which kill off Activas as parts are pricey.
Its not really a case of how often they need eplacing, they can last a cars lifetime (some cases they do as cars a scrapped bacause of poor strut tops) or they could be due for replacement on any Xantia or Xm. They are a bit of a bad design in my opinion, you can spot rust around the strut tops from under the bonnet, if it looks crumbly its not always the end of the world, unless there is little metal reimaining. However its the rubber bonding which can come apart and cause the strut to escape. You know what this means, a new bonnet and a bit of a scare as your driving along!
Bad spheres can start this effect, as they put more force on the tops. If you inspect them and they look good then i wouldnt worry, they should be fine. Have a look under the wheel arch at the too, see if they look good under there. The first sign of one failing is the plastic boot which covers the strut under the wheel arch pops off or mis-shapes as the strut is moving upwards.
Dont let this put you off though, you will probably never need to replace these, just check them on the car before you buy.
Yes the 2.1TD is pretty good, i do like that idea. Citroen didnt bring HDI Activas over here which is very annoying, id love one. Other than importing one and having to drive a LHD car id not be confident in doing any transplants in order to get one in RHD
That green is a nice colour! I very nearly bought a HDi Exclusive in that colour a few months ago, wish i had but it needed a whole heap of jobs doing it just didnt make sense to buy it. Shame.