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.