I'm afraid of both, especially if they're already old.
Its easier to operate these new plastics, fuel efficiency and availability of parts.
With good maintainance, you're easily looking at 10years of use.
camry ni gari safi my bro has had one for 10years in new zealand drove it when I visited him na ilikuwa imegonga 480,000km and it felt brand new never given him a single issue from his advice. the vvti 2400cc version jdm spec.
Any Benz engine is as good as it is, the rest is up to the handler (person behind the wheels) whether M102, 109 or 111. In the M series the best engine ever made was the M119. M119 you can get over 900bhp and 700lbft, twin twinscroll K27 turbos, and ECU control more advanced than 99% of tuned road cars have almost 30 years later.