I've done plenty of really long trips with it (many thousands of km), but I did replace all of the accessible high pressure LHM tubing at some point to get rid of the periodical 'Arret!' light on the dash because the system lost pressure. I can also tell you from experience that trying to drive a DS without pressure is both an art and a body building exercise at once.
Xantia is a great compromise between 'modern' and 'old fashioned', a bit plasticy but the one I had I drove into the ground over many 100's of thousands of km. The final nail in that particular coffin was to lose the headgasket somewhere between Munich and NL and just continuing the trip, it even got decent fuel economy but it wouldn't start up after being shut down and once we looked into the engine it was declared a miracle that it made it all the way back. Great car, all around and given the distances covered ultra reliable.
The real problem in the DS model is the semi-automatic gearbox which is super finicky regarding hydraulics levels (you'll notice a ticking sound when it is trying to engage and can't quite make it due to a lack of fluid, you can temporarily fix that under the hood by forcibly actuating it but driving it like that is really not recommended. Nor is driving it when one of your drive shafts breaks and you block up that side of the drive train. Nice low engine revs though, we jokingly called it 'economy mode'.
I also had an ID-19, a simpler but altogether far more robust car.
Agreed that the ID (idée)-19 had a lot of advantages from its simplicity.
Have a look behind the left hand front wheel cover panel and say that again :) There has to be at least 50 meters worth of high pressure line behind there and a functional analog computer using hydraulics. That's what keeps the car balanced front/back and left/right. The height adjusters just do the static balancing for load, that thing on the left front does it dynamically with a response time in the 10's of milliseconds and keeps continuous track of what 'level' means to the car occupants. It is a very impressive piece of engineering which has some components that will leave you completely puzzled (a tube with a few hundred metal plates in it with a tiny hole through the middle, for instance) if you don't know what they are for.
My Xantia is the Activa V6 model. That ES9 engine is a beast. I've been told that it's usually the car, not the engine, that breaks first with those ones. But really, what I was more interested in was the Activa suspension. It feels like driving a go-kart, yet with the suspension of a Bentley, and the practicality of a hatchback. I know they were trying the same sort of anti-body roll for the DS back in the 1960s, but money was always running out (unfortunately, I cannot seem to find the pictures, I know they exist online somewhere).
I intend to keep my Xantia around for a long time. Took it to the US once, and drove from coast to coast and back again.
DS with a five speed manual is the best combo, keep an eye on your steering housing (those tend to go slowly but failure is potentially catastrophic) and front carrier arms, the bushings will wear out and then you'll start to eat tires. Other than that, your main enemy is chassis rust, everything else can be fixed.
I've recently bought a house with room for a workshop. I've known for a while that the steering on the DS was an issue that needed fixing. Now that I will finally have room to work on it, I look forward to start fixing this among other issues. Including fixing the wiring for the thermostat to the dashboard.
Rebuilt units can be bought, they're better than the originals and should last the (remaining) lifetime of the car, I would definitely recommend against trying to rebuild one yourself, they are super finicky to work on and require a ton of specialist tooling (and experience) to do the work properly.
Note that there are right and left hand drive versions of these. Also there typically is a charge if you don't give them your old one for rebuild in return after you've completed the transplant, and, of course there is a huge difference between the 'red' and the 'green' version, which has to do with the sealing materials used, the 'red' system uses a hydraulic oil which is extremely corrosive.
One example (green, LHD):
https://citrotech.nl/shop/product/stuurhuis-ds-lhm-linksgest...
Obviously, after replacing the steering housing you'll need to have the car aligned again, and while you are at it you may as well replace CV boots and ball joints since it is mostly just the price of the materials at that point.