How do you get a 23yr old 300,000km car more reliable even though they might be doing good?

[car10002]

Hi

how do people get a 23 year old car with over 300000km a more reliable even though they might not be doing too bad when they might not want to sell,

when heaps have already been done and been fixed when they need some suspension work and maybe oneday head gasket or better a low km engine amd a few other little bits to finish them off just intrested to know

And for the people that change cars while there's still value just intrested to know what's the mileage where they loose too much value or start to have a heap of minor problems that cost $100 here $300 there until it adds up to a grand that might make people trade up