Subject: I had complete faith in the mechanical integrity of a fifteen-year-old 200 automobile that had racked up 230,000 miles.
The (subsequently-recommended) M102 engine's head gasket is its mechanical weak spot; nevertheless, this should not cause undue concern if the vehicle has not been subjected to excessive heat. Corrosion however is. In addition to the structural issues, they are known to rust the back panel behind the screen, which may be rather costly to fix.
I came across a couple W123s when searching for "classic cars" on eBay. The one up there was the first that seemed realistic in terms of price, condition, and desirability. Some of the instructions available are in bad shape; I wouldn't recommend them. 'Hard' material seats and a tan interior are standard parts. Personally, I think the blue velour inside (devoid of the faux wood on the dashboard) is one of the most attractive features of the W123. The driver's seat often experiences bolt wear. The sunroof on mine rusted and let water into the footwells, so I really wish it hadn't. Before buying a vehicle, make sure the carpets are dry and steer clear of those with sunroofs unless you're really set on having one. Also, make sure the drain channels are clean.
They are very well undersealed, which brings us back to the topic of corrosion (which is crucial on a W123). An issue arises when the underseal is compromised, since it causes water to be trapped beneath it, leading to excessive corrosion. Maybe it's OK to have one little space. When rust starts showing up everywhere, it's best to just walk away. I'm not sure I trust "full respray" vehicles since, even if they were in dire need of paint, there's no way to tell how well the paint job will be unless the seller is really trustworthy and the price is approaching £20,000.
Getting replacement parts will inevitably become more difficult, so check out RockAuto's website. In addition to listing W123 (but not 230) parts, they have fair shipping prices for lighter products and are affordable overall.
I don't think you really need a completely restored version of a vehicle with legendary dependability, especially because what you're want is probably available for less than £10k. Look around online and stay away from idling vehicles. If you begin at the lowest price point, you will quickly be able to discern what you need and what you can do without.