1.
Swappa is a good place to buy used. Most of what you see isn't someone else's problems, it's "I had this phone for 6 months, and I'm getting bored with it." Some people are like that. Swappa is pretty tight on vetting what's listed.
2. If the phone is locked to TMobile, it'll work on MetroPCS. Unlocked phones are more expensive. If you're staying with MetroPCS (or going to switch to TMobile some time in the future), an unlocked phone is a waste of money.
3. Phones don't degrade over time. Electronics keep working the same unless they stop working. An older phone will stop getting updates sooner, but it will still work. It may eventually get to the point that you want some of the features the newer phones have, but that will eventually happen if you buy a Note 5. Look at Swappa, then look at the same phones at retail price, and you'll see a huge difference.