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 blasé 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.
I can only repeat what I said earlier, based on my current ownership of 5 prewar f3.5 Elmars, 2 Summars and 3 Summicrons (different vintages). I have also owned 2 or 3 Summitars in the past.
Nuimerous examples of the above exhibited haze on internal surfaces, which was cleaned off at reasonable cost by my usual repair service, Newton Ellis of Liverpool, and has not returned. If any lens is left in a dirty state for too long, the haze may attack the glass and cause pitting, which can be dealt with only by a major stripdown (separating lens groups), regrinding, repolishing and recoating. This will almost always not be cost-effective.
However, the only Leica lens with which I have experienced this problem on a systematic basis is the f2.8 50 mm Elmar, which, as I said, has its iris diaphragm located immediately behind its first element instead of between elements 2 and 3 as is usual with Tessars. The fogging I have experienced with these lenses I attribute to vapors from the iris lubricant - the fogging seems to be promoted by storage of the lenses on a camera which is lying down with the lens facing upward. Both the examples of the f2.8 50 mm Elmar which I acquired (as part of job lots) were pitted so badly that I simply had to throw the lenses away. I have not experienced this problem anything like as badly with Summicrons of a similar vintage or even Summitars, which are older.
I have had a problem with mold with 135 mm Hektors - since these are available cheaply, I chose to discard a couple of poor examples and look for better ones rather than consider repair. The only Leitz lens which suffers from serious glass deterioration as far as I know is the f2 Summar. This lens is also often found in poor condition because the glass is soft and does not withstand cleaning.
In short, I wouild say avoid 50 mm f2.8 Elmars - major risk of problems. There is no reason to avoid 1950s Summicrons, these may need at most a straightforward clean and have lower contrast than their modern equivalents, which gives interesting results particularly with color. The modern Summicron-Ms seem to have been recomputed in the days before high-saturation color film so that amateurs could blow their friends' socks off at the camera club with the "superior" quality (higher micro-contrast) of their pictures - with b+w, the contrast of a modern Summicron needs a lot of taming with extra exposure and reduced development.
