Buying a wreck and then restoring it is asking for trouble!
I think it's OK if you have the technical expertise - both on the physical aspect of it, and getting the supporting paperwork to the "legal" stage.
I don't think it is worth doing unless the finished job is going to be worth over £100k - unless you can do it all yourself (and don't value your time). For fun, I've looked at some of the ~£35k TB20s and did some figures on what it would cost to bring it to the level of a late-model one (worth about £140k) and the answer was at least £100k
There are some real shysters about. A friend of mine was looking at a PA28-181 and it turned out it had been sitting somewhere nearly 10 years. It was not worth scrap. But some mug will buy it allright for the asking price of £45k.