You could spend a grand on a modern bike and drop its value by £500.
You could spend a grand on a "restoration project" (POS) and add £800 to its value.
For some reason IME project bikes go for an inordinate amount of money compared to what they are worth fully restored and the cost of doing it properly. People seem to be seduced by the notion of a project and the fanciful expectation they are a pot of gold if you put a few hours in.
That said, that bike is quite a nice example, clearly not a rusty hulk and would be more a case of tidying up to make it nice rather than sourcing dozens of unobtainable parts.