It depends entirely on your situation. If you can afford to do it, what does it matter if the money you spend doesn't go straight onto the value? It doesn't in a lot of cases - if you had an old E-type Jag, the work would probably be more expensive so even though the car is worth more, the result would still be the same.
It's only really an issue if you intend to dispose of the car shortly after you have it restored, or if you foolishly keep a detailed record of everything the car has cost you, and look at it ever again
In a way, it's good to hear of people who are prepared to spend a fair amount of money getting a proper job done. It does sound a lot, but even stuff like your location in the country will make a difference to that. I spent less than that having a lot more work done on the Firenza, and at the time it was 50/50 whether the value of the car would cover the cost of the work, though that's not the case any more. And because I paid for a good job to be done, it still very presentable (IMO) twelve years later.