I have an old multi-use bike (1990 vintage Klein Pinnacle) that has a 7-speed drive train. Out of necessity, the drive train is now a mixture of parts - Suntour indexed shifters, Shimano derailleurs, Sachs freewheel, and SRAM chain. Somehow it all works perfectly. So I wouldn't worry about replacing parts on a used bike. (Check out Amazon for parts availability.)
If the bike you are looking at has been without routine maintenance for 15+ years, the grease in the hubs, headset, pedals, and bottom bracket needs to be cleaned out and replaced. And I'd go ahead and replace the chain too.
JEAN
2011 Specialized Ruby Elite - carbon fiber go-fast bike
DiamondBack Expert - steel road bike
Klein Pinnacle - classic no-suspension aluminum MTB