Just checked mine,
Baldor 3/4 HP 8" wheels, 3600 rpm. Great for tool grinding.
an old Cincinnati, 1/2 HP, 3600 rpm. same thing
Home made one with pillow blocks and a belt drive, probably 1,800 rpm or less, great for wire brushing and buffing with cloth wheels and compound.
If all you want to do is wire brush and buff stuff buy a "buffer" not a "grinder" the buffer doesn't have the guards or rest, or as much of them, so easy to manipulate parts around the wheel.
Grigg