Istars_sister Said:
“[...]Can I simply also purchase a hard drive and a copy of the correct osx? I have a very high level of mac experience.”
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Depends on the Model...
For older Macs, items are upgradable, but it depends on compatibility, of course.
But...
For Much Newer Macs (Locked Interior):
In nowadays’ Macs, every component is made to interoperate with one another. And, they all soldiered in. So, no don’t replace it, if it is non-removable.
Also...
Why would you “buy” a copy macOS? It’s free of download nowadays. The only reason to purchase one would be for old, obsolete versions that you had to purchase way-back-when.