NAS is just a Linux machine with fancy storage.
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(I know thats technically not an accurate statement but Im standing by it, I know what I said)
But for a one-time backup of one pc you just need a disk tbh - and even that one can be the single one in your current pc if you are able to make a partition for either backup or for Linux.
Like, space permitting, just carve our a partition & transfer there what you would to NAS (or external disk drive, or an additional drive connected to the pc).
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If space is a bit tighter just carve out the few gigs needed to install Linux on that (nowdays for most users "it's fine"). Then must boot into Linux & use the rest is the drive as is.
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Ofc if you have full disc encryption, raid etc this solutions are slightly more complicated.