3 examples: One is almost totally original, one is a total restore and upgrade, the last is cosmetically pretty good but needed a few mechanical bits. All 3 are fun bikes, and I enjoy riding the wheels off all if them. Every bike is different, and required a different approach. I don't keep these bikes to sell, these are riders. If you buy them to resell, its a totally different deal. Do what feels good for you.

My 1964 Legnano, had since new as a junior racer, 16 years old. It has lots of battle scars, which is now called Patina. I would never touch it except to polish the chrome and clean the mechanical bits as needed. I still enjoy riding it after 54 years.
My '73 Raleigh Super Course was built from a rusty, spray painted frame I found that needed re-brazing and was missing everything but the bottom bracket and crank arms. It is the first bike I have ever built, cost way too much to finish, but I loved every minute of it. I configured the components just the way I wanted, and enjoyed riding it in this year's Eroica.

Just picked up this '78 Peugeot, with paint, chrome and decals in almost perfect condition. It was missing a front derailleur and had funky brake levers, and crank issues. I spent less on this whole bike than just the paint job on the Raleigh above.