Okay, now that I'm searching around a little more there appears to be some less expensive replacement batteries out there. Are these any good or should I steer clear? Is there any advantage to the ones on iFixit over a DIGEO, Anker, etc. battery that are all over Amazon?
Typically you get what you pay for on batteries. Beware of very low cost....
One third party I can recommend is
NewerTech/OWC. Have used a few, and they have been as good at the OEM Apple batteries (run time & degradation over time)
They also have easy-to-config and order RAM and SSDs, plus tools. Perhaps not the cheapest you can find, but no questions about quality, support, or returns.
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As for eBay....I am sure lots of stuff is fine, and a good price. Problem is, you might get someone off loading damaged, unstable hardware. It can happen anywhere you buy used, but eBay by it's nature of being anonymous makes it too easy for folks to sell damaged goods deceptively. The risk is higher (in my view) that you can end up with someone else's lemon.
Based on this....I would do the upgrade(s) on my own (known condition) machines first, and go to used if/when it is not cost effective....or if there is a bigger issue (logic board for example)..
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Last thing: Good advice farther up the thread about replacing the SATA cable.
There were a batch of MBPs about this vintage that had bad cables. They would fail for seemingly no particular reason, and mimic a failed HD. Luckily it is easy and cheap to replace.