I'm not really into the GLC so try the MB specific forums. MBworld.org and Benzworld.org. I'm more into the E class. Lately they've been better than in the past. I have the parktronic on mine, but not the park assist part, sensors on those can go bad, but it doesn't seem like a very common problem or not too many people have that option. Lighting package is pretty good. I have the bixenons which are actually brighter than LEDs, but not too many cars had that. It also had the active curve illumination where there are motors in the headlights that swivel the beams left or right up to 12 degrees when turning the steering wheel. The LEDs can do the same thing. Panorama is a love/hate, some love it, some hate it, slightly less headroom and the shade isn't dark enough become it's somewhat like a cloth cover in a roller instead of a solid cover like the regular sunroof. I love mine though, makes it feel more open and lets in light, I usually leave the shade open. Can make noise on bumps eventually as it seems the bolts still loosen. Get android auto/apple car play. Those are probably better than the navigation package. While mine is older, I still have new maps on it and it's still worse than google maps. Some hate the implementation on the 2017 E class. The driver assistance package is also pretty popular. Love keyless go, here's how MB has gotten better, in the previous generations, the door handles had buttons on them for locking/unlocking. They were basically covered by a rubber membrane and eventually water got inside and corroded the contacts. Bingo, that was a $200-$300 door handle plus labor at the dealer so that was about a $500 repair. Now the sensors are sealed in the handle so water doesn't get inside, haven't read of a door handle failure in the 2010-2017 E class.
In terms of warranty, you can get an extended MB warranty for up to 7/100k as long as you still have the original warranty, on the E, they were in the 2-3k range depending on how long and which vendor you bought it from. There were vendors listed in one of the forums I mentioned earlier that will discount it instead of paying the list at the MB dealer.
And yeah, depreciation is massive on them which is why I liked to buy them used. All the toys without the price. They can be very solid, very quiet cars that are fun to drive. Love mine.