Installation isn't as straightforward and as easy as in another systems because Apple decided to keep pushing the usual ARM bullshit of not including a proper UEFI with the system;
Some stuff will be broker, but you most likely don't need it for self-hosting;
If you keep macOS around you may have good luck with virtualizing Debian using UTM or VMware. Debian's arm64 images will run at optimal performance on that hardware.
If you're about to spend money I would grab an HP Mini unit with a "T" CPU, those will downclock really hard and you can get a i5-10500T (on ebay) for around 250€... and everything will work fine out of the box. An i7-8500T model also sells for 150€ or something like that.
Have a look at those CPU benchmarks (last one is probably yours):
If you're looking for power efficiency the newer CPUs are always better. Those mini units will downclock and idle at around 9-12W depending on hardware configuration but Apple should be able to do better - at least assuming you've power management working.