Most serious and costly: 1 cylinder lost compression at around 160000 km. The details of that I have forgotten, but it was a pretty costly overhaul altogether (independent garage; we never went back to Rover/Jaguar Duesseldorf after the 1st regular maintenance that had the most unregular price (1200 DEM!).
Else, and probably more common:
De-Dion Rear Axle hardened gradually to the degree of nil suspension. VERY SPORTY :-). VERY bad for your vertebrae :-)
Sorry, can't give you the exact reason, anyway, this was eventually mended for around 500 DEM (250 Euro).
The power steering started to leak, noticable by strange noises when putting load on the pump. Was compensated for a long time by filling it up more often.
Alloy wheels are prone to rust. Should you (I doubt it) still be in possession of an SD1 with uncorroded alloys, protect them against salt and dirt! The corrosion undergrows the varnish, looking very ugly. Same goes for the (fake) chrome stripes around the body: these are aluminium with plastic cover, which wears off to let the aluminium corrode.
Body paint (metallic silver) lost its shine plaque by plaque on the roof after only 7 years. Reason: transparent top layer came off. 500 DM for new paint job.
Minor bump into another car: the bumper plastic instantly disintegrates into the fibres it was baked out of. Very poor quality.
Our own fault: Trip computer died after a jump start. Yes! Finally no 80+ instant consumption figures anymore when putting the foot down :-)
5-speed manual is not very sturdy (2nd gear wear)
First of all: GREAT looks! This is a design peach. Massive, somwhat ducked, although not low at all, stretched, but not thin. Not the elegance of a Jag, but a bit more of the Ferrari/Maserati sportiness. Aside of Jaguar, perhaps the best design that ever emerged in the UK. Personally, I find the Mk2 a little more elegant than the Mk1, and the Vitesse a little more elegant than the VDP (the lowered suspension makes it look more compact, sporty), EXCEPT for that ugly huge Vitesse spoiler.
I have driven the VDP for over three years, together with my father who owned it (and paid for it :-) ). Overall, the quality of the late SD1 model is okay. In those 8 years, rust was no issue except for a spot at the wind screen lower rim (corner) which admittedly is an ugly place for rust, and a small one next to the sliding roof. Doors and wheel arches were still fine. However, the paint quality at least of metal silver is below average. Interior quality is so-so, british average of the time, I suppose. A little squeaking here and there; when you pull the door shut you can feel the trim moving. Carpet seems loose in places. Leather seats are leather only on top, sides and back are plastic. Good quality though.
Gear box: at 160000 km, you can feel every tooth of the gear wheel when shifting into 2nd. Still working though. It is known that the manuals are not sturdy units, in contrast to the auto ones. On the other hand, they are more economical and allow higher top speed.
Space: for a car of this size, it is NOT very spacious! With the standard Mk2 sun roof, head room is nothing for the above-6-foot-fellows. The back definitely lacks leg room, and for tall people headroom, which cannot be compensated by a nice leather bench. Leg room in the front passenger seat is impaired by the protruding dashboard, and both front seats could go back a little more (provided no one tries to sit in the back then).
The boot is very spacious, only the high tail end of the Rover makes this a bit of an exercise to load.
Sound: the sound is great, but too muffled! Since this car is from before sick noise and pollution emission laws, I certainly would fit a y-Pipe or equivalent that brings this excellent V-8 sound out to the open! Look at it like this: since you pay extra for having no cat converter, you should take the opportunity to at least have the sound that no modern car can have! Ay?