It needs to hold a seal... A couple of my cornies look beat to hell, but they hold a seal. One of them had a lid that was obviously banged repeatedly with a hammer. So I bought an extra thick o-ring
and some keg lube and it works great now.
Seriously, as long as it holds a seal and you replace the o-rings on it, you should be fine regardless of how many dents and dings are on it.
As for the rest of the system, I would say the regulator is the most 'fragile' piece. The CO2 cylinders are usually ugly steel and work just as good as a shiny new aluminum one. The most important part of the whole shebang is going to be secure connections with all of your tubing to ensure there are no leaks. Don't over-tighten the regulator on the cylinder or you'll crack the plastic washer and have a slow leak.
Keep your beer lines clean and clean regularly. I replace mine once a year too.
It all seems complicated, but really its pretty easy once you understand how it works as a whole.