I have an old Mac Pro that has been collecting dust for years. Today I bought an SSD and install Debian on the machine. It works flawlessly.
Further reading revealed that there is an active community around the classic mac pros and thanks to their modular nature they can be fully upgraded. People even upgrade the CPUs in this thing.
So if you like playing around with a PC like the old days, that is aldo Linux compatible, a Mac Pro 5,1 seems a good choice. AFAIK you can get it for cheap and a decent upgrade won’t break the bank.
Depending which version of the MacPro you have exactly, that machine from 2010 is around the speed, or slightly faster, than a Raspberry Pi 5 with 8 GBs of RAM these days. The problem is the energy consumption, not really a green machine to run.