I would stay away from 50's Leitz lenses - I have had lenses with the 'etched glass' problem - it still cost $100 to get them taken apart and cleaned to no avail. The problem is no recent discovery and has been known for (at a minimum) 20 years.
You may find a 50's Elmar/Summicron/etc. that doesn't exhibit etching but it is a matter of time and humidity until it does. Not all lenses used the glass formulations that were prone to etching but the early 50mm f3.5 and f2.8 Elmars were a real problem.
The 80's Summicron is probably the best buy if you want Leitz glass. I am rather blasť about Leitz optical performance: I can't honestly tell the difference between an 80's 50mm f2.0 Summicron and a comparable Nikkor - even with TechPan and a resolution target. There is nothing wrong with the Summicron, but just about all major 50mm f2.0 lenses are superb performers. OTOH, if I look at the build quality between a Summicron and a Nikkor (or any other modern lens) there is no contest: the Summicron wins by miles.
The advice to seek Cosina/"Voigtlander" lenses is good advice if you are going to use the camera for making photographs and are looking for the best bang for the buck.
For me the difference is in the bokeh.Nikkors are more prone to 'cross-eyed bokeh'.
The 50 2.8 Elmar is one most inclined to etching but the elements are available and can be replaced.