If you're referring to the Mark II, specifically, I think cars are worth far less with updated parts, if the original-type replacement parts aren't included. I know of only one Mark II that may have all of it's original parts. It only has 6,000 miles and the guy doesn't want to sell it. It hasn't run in 50 years.

If you do a brake upgrade, keep the old stuff. Keep the old engine, especially if its serial number number is what the car had. If you can make a modification that's reversible and it makes the car safer, I'm all for it.

I think the only real modification that will increase the value of the car is seat belts and shoulder harnesses. I figured out a way to do both.

Cars are used cars the moment they leave the factory. They either cost the manufacturer or the dealer carrying costs and the new car owner depreciation so most cars are a declining asset. Our collector cars are finally gaining in value. If you're a collector that bases their decisions on what their car might be worth, keep it totally stock. If you use it and modifications make it easier or safer to drive, that's great, too.

I never bought a car for its resale value, so maybe my view is skewed.

After just finishing a last nut and bolt mechanical restoration on a friend's Mark II I can tell you I'm glad I got it stock.

BTW, if you do everything on a Mark II restoration and bring the car back to original specs you'd be surprised how few improvements are really necessary. The owner is extremely happy.