What do you consider as "modified"?
My first computer was an 8 bit computer (Sinclair ZX Spectrum +), like most of my collection. When it was build, it had some components (axial capacitors, 7805 voltage regulator, 4116 RAM chips, the ULA chip) that now are rare. There are some Spectrums fitted with more modern capacitors, some have replaced 7805 with other voltage regulators and others have replaced the 4116 chips (that use +5v, -5v and +12v) with other chips that use only +5v. Even some computers with defective ULAs (it was a sinclair custom chip) have been modded to repair some failures.
My ZX Spectrum now has a composite video mod (because it had RF video output and modern TVs often do not pick that signal correctly), and I'm going to put a little heatsink on the ULA to extend its lifetime. Do I consider it as modding? Well, I think about that like repairs and maintenance needed to keep it running. If I don't do some mods, it will became a retro paperweight.
Some might apply to some modding made on other computers (enabling RGB or composite video output on RF computers, RAM changes, etc), and I have no problem with buying repaired/modded computers. I have a few exceptions like external changes (like replacing original keyboard with custom keyboards). And, of course, I would not buy anything with mods made to overclock the computer.
I have traveled across the universe and through the years to find Her.
Sometimes going all the way is just a start...
I'm selling some stuff!