maybe get an old lappy save electricity?
Of course if you already have an old laptop lying around. A 1L PC also does the job well if it has those "T" series processors - Dell Optiplex, Lenovo Thinkcentres , HP Elitedesk 800 minis etc. you don't need a modern cpu, just some old x86 boxes with low power consumption since you'll be running the Pihole/Unbound @ 24/7/365
And if you are ready to explore, and add more services on top of your Pihole/Unbound eg. Tailscale, Headscale. If you want room to grow then maybe at least an i7-8700T or i7-9700T for slightly more headroom. It really depends on your interest. For 90% of users who just want to run an ad /telemetry and malware sites blocker, even a Pi Zero 2W would suffice. But must do a bit of maintenance once a while lar.