Urrrmm... well, with all the work I have been dooing on them last year the WORSE areas for rot are the following and NEED checking carefully!
FRONT: Get the car started and put it on top height. Front end of the Cill, check the jacking point plate.. IF its bent inwards which they useually are, check further inwards behind the cill at the "OutRigger", they rot here!
An Example after I welded a plate in...
BOTH sides of the car, also behind the wing where it curves down. You wont be able to see without removing the wing though!
at the back, the ENDS of the Cills on the outer side, next to the rear jacking points. A bit easyer to spot.
At the back UNDER the car, DO NOT VENTURE UNDER WITHOUT SUPPORT! Check the rear subframe mounting points too.. Ive had a bad one in this area..
Those areas seem a little less frequent, but alas they DO go.
Front and rear subframes can and do rust, check them carefully! Front especially, its a full size O shape frame that goes right round the engine bay, the front section can rust away and get holed!
SERIES ONE CARS: Options are a 2.0i 8v NON Turbo Engine Petrol, a 2.0i Turbo 8v engine (fairly common), a 3.0 V6 12v auto (though manuals were offered in 1989 but scarse), and of course 2.1 TD engines. Manual on H and J, then option of Auto or Manual onwards, though almost all were Auto. Though Ive had a few Manuals pass by me.
Reliability. The 8v Petrol engines are a cast block alloy head. Rather tough, but useually like ALL XU engines need a head gasket at about 100k. Most should be on their second one now and not need one. The same for the 2.1 TD's too.
Petrols a bit Thirsty, 28mpg or so, where as the manual 2.1 TDs are upwards of 45+ mpg.
Autobox's. IF one is fitted DONT BE SCARED. Firstly be sure it works correctly and changes up and down smoothly, if it does, it litterally needs a regular 6k fluid change with DEX 2 to live on. A mate has a 2.1 TD with 250k odd on the original box, but I do a 6k ish fluid service on it!
LHD displays always work on S1 cars.. Dont panic too much about a continuous Brake Bulb failiure wanring, it may just need an extra earthing wire from the lamp unit to the chassis.. ABS is an issue, and has common problems. Though again on here theres some GOOD topics on ABS.
Windows can fail, useually the cables, but also the switchs can play up too, a swap can sometimes sort that.
Strut tops need checking too.. NOT so much under the bonnet, but more under the front wings.
Here is an example of a SERVICEABLE strut top:
Its not perfect, but is perfectly safe. The base plates do rust, but you can clearly see the rubber section. This should be flush with the plate, if you can see the cone pushing DOWNWARDS they are on borrowed time. Though they are available on exchange now for £100 a side with a fourm member so theres NO panic there now (They were £300 quid a side from Citroen)
Those are the worse points I can think of off the top of my head, no doubt others may come forward with other opinions or disagreements with me. But with having worked on about 10 or so XMs Id like to hope I have given good advice for finding a good one.