Buy S-Class!

Hello, today I saw a S 260 Bj 1986 at a used car dealer and suddenly fell in love with me. However, I would like to ask you what I have to pay attention to when buying or how the tax from the car beats to book etc. How many km would the car keep roughly estimated because it has already 288000km down but still looks so great. Thanks for the answers Mfg Dickworzkarl