Given it just cost me half of what that car cost just to put in a ZF gearbox, I'd say it is worth the money, if genuinely good. Not everyone is handy with a spanner and it only takes one major issue to make an average car more expensive than buying a good car.
This is of course why we need prices to rise. I can sink a few grand into my car every so often because I write off that money as funding my hobby. I don't smoke, have no interest in the latest iphone, don't drive a posh modern car nor own a set of carbon-fibre golf clubs. To be as good as a £6-7k car, I'll likely need to spend £12-14k over time. But it runs well and keeps me happy in the meantime plus I get the knowledge that I save one more example.
Many others can only do that with one eye on what can be recovered if they sold on and that's a problem with a car like the P6. Many better examples seem to take forever to sell. I know of a 2200TC which has no welding and drives like new for £5k which has been for sale for nearly 3 years. It might be the best, unrestored example in existence. Certainly in LHD form. But no V8 is no sale in the European market.
If I were to do this again, I'd buy the best, non-customized (not necessarily completely original as I see no reason to build back in the limitations of the time for a car which is used) car money can buy, even if it were 5 figures. Let's face it these cars should at the top end fetch £15k given what some desperately ordinary and crudely engineered Fords make.
It's cheaper in the long run.