The ELCB won't help in the event of a lightning strike or spike on the power feed, but a UPS would. I really suggest you get one - they'e a pretty inexpensive way to help protect expensive electronic equipment.
As for the age of the computer and how that impacts 'robustness', the reality is that over time electronic components, particularly capacitors, 'age' and drift out of their original specifications. It doesn't impact on chips and ICs so much, but can weaken power supplies in particular. Powering down, and up, can cause fluctuations in voltage through power supplies that can cause them to fail as their components age, but for quality-built electronics, including Macs, that isn't generally a particularly significant issue.
Just as important, or even more so, is where you live. If you have very stable supply voltages, you can expect much better long-term reliability than if you have lots of power spikes, brown-outs or random outages and reconnects. A UPS will help with these issues too.