Barn find Anyone ?

.. fully restored bikes - yes, I know quality may be variable - aren't much more than barn finds. If you like a challenge and don't mind spending money then a restoration project is fine. Personally I enjoy the challenge and am currently working my way through a BSA C11 which has components from at least 3 different models. Keeps the brain cells as well as the technical skills active.

Partly might depend on what you have in your spares collection.  

Barn find, basket case or runner? Have had all three.
On barn finds, it depends on if you can figure out the reason why it was left. Sometimes due to a mechanical failure, sometimes just stored and forgotten or owner had other more important things to do. If you knows why, easier to figure out if it is worth to buy.
Basket cases usually was taken apart for a reason. Often missing some unobtainable parts.
Runners usually need some fixes. Lots of poor mechanics out there.
Personally prefer runners as they usually need less repairs.

If it was last one the road 40 years ago, will you have the original registration? - part of the value can be a transferable registration.