If I buy a refurbed MacBook MF840LL/A will it hold the most recent software?

"...will it hold the most recent software?"

Current, yes. Nothing newer. See this Apple page:

macOS Ventura Preview - Apple

If you scroll down, you will see the next version of macOS, "Ventura," will release this fall and will not run on Macbook Pros older than 2017.

I would not buy a 2015 model unless you are OK staying on Monterey.

It'll run Monterey, and software compatible with it unless otherwise noted.

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