Best used systems are the Lenovo ThinkPads, T models for the mid size models.
T430/440/450, maybe the slim s versions, with a new solid state and clean Windows setup is my favorite laptop setup for under $500. I don't know anything new you can get for under about $500 will have the same quality or reliability as a 5 or 6 year old ThinkPad. My main system is a W520, those models came out in 2011. 4 RAM slots, plenty of space for several drives for storage, runs like new. Only catch is finding good batteries for them, you may need to spend another $50 or so for a new Lenovo battery.
And don't post a bunch of threads on the same topic then reply every 10 minutes, compose your thoughts and post once. We try to keep the forums pretty cleaned up from duplicates and wasted space. Edit your first post don't reply to it unless you are replying to someone else. You have over 60 posts and about 8 new threads about the same "looking for laptop" topic, stick with one, do research, don't just stick every random thought you have on the forums.