I have a Baldor 7" 1800rpm commercial grinder that I bought at auction for $60. Wonderful machine. It is my most treasured power tool, even in front of my Delta Unisaw.
I find the MOST important thing in a grinder, is cast iron tool rests. If it has sheet metal tool rests they get bent and are impossible to keep square to the stone. Look for a model that has good tool rests.
I had a 6" Baldor hobby grinder. I burned it out. Baldor makes junk too.