Right out of the box I’m sure it’ll look fine. The trouble comes months down the line when it begins to lose capacity much more quickly than an original battery would. Watch the capacity with a program like Coconut Battery and be careful that the warranty doesn’t run out.

Ifixit appear to be sourcing theirs from the same place as everyone else. It’s not like there’s a secret “good aftermarket battery” factory and only they know how to source from it.

The only drawback to a third-party battery is that they are all garbage now. There was once a time when good third-party batteries were a thing, but that time is gone.

Upgrade as many internal pieces as possible. 16gb ram and as large of an SSD as she needs (assuming both of these can be upgraded).

2014 MBA RAM is soldered to the main board and cannot be upgraded.

The MBP changed a lot from 2011-2015, so this is kind of a broad question. At the start of that span they were the thick and heavy unibody models with optical drives, 2.5" HDD/SSDs, and non-Retina displays. By the end they were significantly thinner and lighter, had Retina displays, and used flash storage. Also over that span the latest-supported version of the OS went from High Sierra on the Early 2011 models all the way to Monterey on the 2015s.

For a unit in proper working condition (no damage to screen, no missing parts, battery condition reporting as “good”, only ordinary scuffs and scrapes on the exterior) I’d skip the 2011s entirely, go maybe $50-75 for a mid-2012, and up to $200 (and not a penny more) for a mint 2015.

And that’s only if I needed one. Which I do not. I have plenty right now.

Third party batteries are all poop. All of them.

Find a nice used one with a low cycle count on eBay or similar.

Here’s an adapter card for $20.

Nope, the Intel-based MacBooks from 2016-2020 are cursed with display issues (“flexgate”) and the butterfly keyboard, among other issues.