In my life I have financed the restoration of multiple cars and in none of the cases this resulted into an economical gain for myself. The truth is that there are very few cars where the enormous cost of a full restoration really can be justified with increase of the value of the car. You probably couldn't even restore a Lancia Stratos (nuts and bolts) for example for the total value the car fetches today. Of course it all depends on with what you start with. But I have seen people spending many hundred thousands of dollars to get a car right. And in my cases the restoration usually equaled the value of the car plus/minus, but I also had to buy the restoration basis on top. Why did I still do it?
1) it's fun to see how a car rises to its old glory
2) you know what you have as you have observed the process yourself
3) your connection to the car is much tighter as you know so much more about the car
4) the process as such is interesting and you get to know many people you need to ask for help or part or so
5) you preserve an important piece of history
6) you actually create local value add for specialists, something to feel good about in the times of financial recession
7) you can influence the end product, be it by the choice of color or by selecting materials or refinements (if you have to, but that's another story)
So, I will do it again, if I can afford it and I usually don't regret it.