Sadly, this kind of rot comes standard with UK cars, a friend has 7 examples undergoing restoration at the moment and the 5 from the UK are all completely rotten, inner front wings (fenders), boot floor , sills (rockers), front cross members, rear of front wheel arches, floor sections and rear suspension members.
The cars all have to be mounted on a spit/rotary jig and have new sections welded in. This is a very time consuming and expensive job as to do it properly involves totally stripping the car of all mechanical and trim components and leaving just the body.
The truly terrifying thing is that in the main the cars looked at least 8/10 cosmetically and one particular car a first year 250SL drove and handled beautifully and appeared to be a true survivor. When it was closely examined it was in a terrible state underneath. It seems that all UK cars that have not had a full restoration should potentially be considered as massive restoration projects - Of course there will be exceptions and carefully preserved and cosseted examples do exist but in my experience the vast majority of even dealer sold sub £30 K cars over here are in need of serious work to make the underside match what the car appears to be on the outside. These issues are not merely cosmetic but safety related. The problem is that everybody on here probably knows this already but people attracted by recent publicity probably do not ! I have never known of another car that can look so good and drive so well whilst being riddled with structural rot !
If you have the time it may pay in the long run to buy a "basket case" with good trim and cosmetics with the intention of tearing it down and repairing it properly rather than buying an unknown "restored" example from an unscrupulous dealer. Of course there are reputable dealers and repairers as well as private sellers but these cars, perhaps more than any other, NEED to be inspected prior to purchase by an expert and on a car lift.
My 230 cost me £14K but everything was wrong and I spent a further £30K over 2 years having it properly structurally, mechanically and cosmetically rebuilt and or restored. I may be able to sell it for what it cost me to do (not that I want to) and have been offered £50K for the car at two different shows but the main thing is I know it is right and safe and will stand the test of time, I have a full photographic record of the restoration and the fact that I project managed the whole thing gives me great peace of mind. I have met several unfortunate people who have bought cosmetically attractive cars for £30K which need the same amount spending on them as mine did which is why I bought a known "knacker" at the outset !
Steven.