I bought a used G26.3 several years ago, my first Glock. I cleaned it, lubed it, put an SCD on it, and started shooting it. At ~325 rounds, I put a set of Trijicon HDs on it and kept shooting it.
The *only* issues I’ve ever had with it were several Russian steel case rounds that didn’t want to pop, one aluminum case from an Independence round (noticeably weak ejection - I stopped and checked the barrel for an obstruction) that flipped over and landed in the ejection port, and after about 2500 rounds, a chunk of the RSA nose cap disappeared. One $18 new RSA later, and it was back online.
It is one of my favorite guns of all time. I plan to get a gen 5 at some point, but really I’m not in a hurry. This gun has killed every critter I’ve shot it at, hit every target, runs and runs like anything using quality ammo, and acceptably well on the cheapest crap imaginable.
What more could I possibly want from a gun?
Sometimes, I confess wanting a longer grip. That’s when I get out the 19x.
I would probably not replace any internal parts unless the gun doesn’t run. The odds of it running right out of the box are definitely in your favor.