When was the last time it was updated or maintained? Almost positive it would be old scaling pipes at that age, so it would catch things easily and cause it to back up easily or at increased risk of collapsing
I'm actually a plumber tc, if you have any questions I'll try to help if you want to pm me. I can guarantee your plumbing isnt original though unless you have like cast iron drains or some crazy old shit and even then it would probably have cracked by now.
Biggest thing to look out for in old houses is the foundation and the electric. Make sure its not knob and tube.
Best advice I can give is hire a fucking home inspector. Dont try to save $500 for peace of mimd on a $100,000 purchase.
Advice from an actual plumber is fucking amazing, I'm just going off of personal experiences.