The Summar is an amazingly good lens. Many Summars started their lives uncoated, but were coated post war. The front lens is made from very soft glass. Coated Summars are alas often scratched too. Try to get an unscratched coated one.
Do not clean the lens with an old handkerchief, but use a soft cotton swab, only to wipe off the dust. Keeping the lens clean is better than keep cleaning it. When shooting, do not use any filters, but use a shade. The Leitz FOOKH (a short FLQOO) is the best shade for the Summar. A FLQOO will vignette.
When cleaning a Summar, use a binocular watchmakers loupe to see what you are doing.
When you look through the lens from the back against the light, you will see all the damage on the front lens. Do this before you decide buying it or not. Be very, very careful when cleaning a Summar.
Erik.