I was having issues so I took off my alternator and brought it to Oreillys where it failed their test confirming my suspicion. The old one was OEM and lasted 247,000 miles. Oreillys wanted $220, so I went home and looked on RockAuto for comparison (appx $100-150 plus core). My questions are:

-New or remanufactured?
-Which brand should I buy?

(Alternative) This Pathfinder is no longer my daily driver. I looked online at my local PicknPull and saw they had a 2000 Pathfinder in stock and a used alternator is $27 plus $7 core charge. There is a chance the car could have a slightly used alternator (replaced it before it ended up in the junkyard) and not the original OEM. If it does I could buy it, take it to Oreillys and get it tested and install it. If it fails I could return it for in store credit. Does this sound like a BAD idea?

Thank you for you help!

It's a value judgement all yours. What have you got to lose? Time? Money? Pease of mind?
Bosch are decent if made in Germany. Hitachi and Mitsubishi are premium choices as well.
Remanufactured, actually remanufactured and not just rebuilt, alternators are a good value.

UPDATE!

The junkyard had a 1998 Pathfinder that had just arrived (the 2000 one was filthy all over) and was in excellent looking shape. It also just so happened to have a sticker stating it had a timing belt change in 2020 at 128,000 miles. So I took a chance and retrieved the alternator, and it passed Oreilly's machine test with flying colors. It was a slight pain getting it back on my vehicle, but I finally did.

I started it up and the belt was SQUEALING bad, without stopping. I loosened it a bit and it seemed to fix the squeal (I turned the car off and back on with no squeal). Next morning I started the car to a squeal that lasted about 5 seconds so I loosened it a slight bit and started the car again with a squeal that only lasted about 3 seconds, so it was getting better. My question is should I keep slightly loosening the belt until it stops? Is there something else that could be affecting it? The belt itself doesn't feel too loose or tight, sort of right in the middle.

Also Since I've been dealing with this alternator issue it has caused two batteries to test bad (WalMart replaced both with no question). Is it normal for a failing alternator to kill a battery?

Thank you for your help!

Squealing belts typically indicates it's worn out or that the belt does not have the proper tension. That can be too tight or too loose. If you are using your old used belt on that salvaged used alternator, it might be squealing because the old belt was worn into the original alternator pully. That would be me putting on someone else's used shoes that are my size but they feel wrong because they were not worn and conformed to my feet. Loosen it up a bit more if that seems to help. If the squealing doesn't stop, buy a new belt, if you are using your old one.

A bad regulator in the alternator will not charge your battery so yes, a failing alternator can be the cause of a battery losing it's charge.