Dear Marco,
You certainly choose the right machine. I recently read that the Mac Mini with M4 is fast as hell. I read reports where people bought the cheapest version and upgraded it with third-party hard disk and memory modules. So, good choice. However, as you know, it comes with Apple Silicon M4, and Parallels does not support running virtual machines created on an x86 computer! So, forget to grab your old virtual Linux (like,e.g., OpenSuse 11.3 or 42.3) and install it on the new Mac Mini with Apple Silicon. Parallels will not execute it, though you can access the virtual hard disk of the old Linux in Parallels on the new Mac Mini but Parallels does not allow you to have more than 5 virtual hard disks installed! I recently read that the Parallels business version (which I have) allows running an x86 image, but it is heavily experimental and very slow, and success seems unguaranteed. Indeed, I did not try it myself. Perhaps someone else here did and can report about it.
Also, installing an older Linux, which was developed/compiled for x86 CPUs, will not install on ARM64-type computers. You need an ARM64-based Linux like OpenSuse 15.6 Leap for ARM64 computers. Another possibility would be to allow Rosetta in Parallels Options. This allows the execution of an x86-based binary program on an ARM64 Linux. However, this worked for me only for programs that do not require a shared library (as the shared Libs are all compiled for ARM64). This is because when you install ARM64-based Linux, there is no parallel world available with all the x86 shared libs installed (and I did not figure out how to do that so as to keep both worlds with their shared libs 100% separate -- it might be possible though). So, if the program you need to run is your private homegrown software, or you have the source code available, you may be able to compile it as a static version using compiler options (so that it does not require shared libs). If you manage this, you can run it in OpenSuse 15.6 Leap by enabling the Rosetta Mode in the Parallels options. Although there are notes in Parallels that execution might be slower if you enable Rosetta Mode, I did not recognize differences in having it enabled/disabled. Best wishes and good luck.