I recommend against the Pi Zero. Once you add in the cost of the microUSB to USB-OTG adapter and the ethernet USB adapter you might as well buy a 3B or 4. Price aside it adds an extra mechanical point of failure as microUSB is not very robust.

RPi5 is definitely a huge overkill. Plus, it needs a power adapter, probably some cooling, and some space to seat it.

Pi Zero 2W + micro usb ethernet adapter works perfect for Pi-Hole, and has an almost invisible physical footprint: Small enough to hot-glue on the back of your router, happily runs with power from one of the router's USB ports, and you get a 10cm ethernet cable to avoid network cable management.

If you want a machine to run 24/7 for a long time, running it of an SD card is a bad idea. The NVMe support on a Pi 5 is important for somthing like a PiHole

Oh, it will definitely work on older ones. The one I have, w/o logging in and explicitly looking, is a 3-$SOMETHING, probably 3b+. Works just fine.

My Pi-hole runs on a ~13 year old Model B, which has survived several house moves. Definitely don't need top of the line hardware for it!