Ceramics are tougher than people think. Not that you should deliberately be smacking them and dropping them or getting hit by a car while wearing them. Those plates are gonna be on you as a part of you, so treat yourself with care. Much like a car, you take care of it and it will take care of you for many many years.
Even if there’s a crack, a well made ceramic plate should be laminated with strong adhesives and thus the plate will hold together enough that there wont be much significance in performance degradation from a crack.
As for age, it shouldn’t matter so as long as they weren’t horribly abused and neglected. There are Vietnam chicken plates and 90s era ranger plates that stop their intended threats.
100% PE (UHMWPE) plates are not really the way to go. PE is not known to reliably perform well against stuff like M855, so you’d be playing with fire regardless if they’re marketed to stop it. However, certain PE plates may be applicable in certain circumstances or requirements with some caveats.
There’s not much reason to stick with steel these days. It’s a obsolete body armor material with risk and downsides outweighing the benefits. In any case, go for multicurve ceramic plates. The money you don’t spend on steel plates and rifles/accessories that only see the light of day a couple times a year helps invest in ceramics.
As for helmets, just stick to a reputable US manufacture such as Opscore/Gentex, Highcom, custom armor group, revision/galvion, whatever flavor of milsurp ACH/ECH helmet as long as it’s not compromised and you put a upgraded helmet liner than standard ones.