The hydraulics are pretty much the same as the Xantia but this xm will not be Anti sink, its a real citroen

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This is also a MK 1.5 and as such is the best age to have as you get sports and auto mode suspension and not auto and slightly more auto mode, the windows dont drop out, its got the citroen dash not the one that was pinched out of the pug 405 and the suspension doesnt pack up and give a hard ride due to diode issues.
you will not be dissapointed with this car as the owner is known here and is in fact the XM Columnist for the CCC magazine, he is an honest bloke, a bit shady, but honest
As with any Car though check the fluids are clean, that everything works, brakes sharp, steering tight and light, no abs lights, also make sure the suspension is nice and soft and the ride height looks good.
If its an auto make sure it drives through all the gears reasonably smoothly and that there is no jerky engagement going between R N and D, also check that the 3rd to 4th shift is smooth and the revs dont go shooting up between changes.
Like i say though all this is unimportant, it will be a nice car this one.
D