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Is a old raspberry pi viable for anything with Linux?
(programming.dev)
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If you want to tinker with a terminal it should do it just fine.
The Pi Hole part is still possible, though. If it is a crappy ISP router then it into a gateway and grab a cheap router on marketplace to do the real work.
Actually, if you are just planning to figure out the basics and not do heavy lifting with it, it would do both just fine. You could learn a bit by getting it configured and keeping it up.