Lifespan of light fixtures or lights themselves?

I wouldn’t honestly worry too much these days - and for a very good reason. Before, many light fixtures were powered by bulbs which, dependent on the fixture and bulb type, had an average lifespan of a year or a couple of years before they went out (or got so dim you couldn’t really use them anymore). Nowadays, almost all light fixtures are powered by LEDs which have a very long lifespan (>10 years on average), and they won’t explode if you accidentally get a drop of water on them when doing water changes.

If you are buying used light fixtures, I’d try to avoid high-powered units since they must have fans or the lights get too hot and will burn out, and I would also avoid units with non-common light bulb types (which means replacement bulbs are pricey as they are uncommon at best).