One of the great things with a Glock is they were designed to be simple. They don't have 190 parts, etc. Even as an armorer it is rare or an exception rather than a rule that we see many "broken" glocks. If you do your stadard cleaning/light lubrication, and if something feels out place after cleaning it, bring it to an armorer to check out trigger spring (very inexpessive to replace), recoil spring, and clean/check the internal components. Most armorers are usually making a modification that the user requests such as a lighter trigger, new sights, etc.. I have a Gen 1 G21 that still with all the rounds, shoots great. I've done a few detail strips just for cleaning and inspection. But I've never had to replace anything (other than swap out sights) to it... (actually I did replace the recoil spring a couple years ago)
Open box, grab a box of rounds, get some hearing, eye protection, and shoot safely, and it will be a reliable firearm for years to come! Thats the beauty of a Glock!
Oh and for any new Glock users (first time), that IS NOT rust you see in the slide. I'm sure there are other armorers here that has got that glock from a user saying it's already rusted... It's glocks packing grease and will come off with a box or 2 of rounds or so (some clean it off first, I just shoot it and it comes off, but everyone is different)..
And have fun with your new glock!