If you have the resources to build or purchase a variable speed 2×72 belt grinder with a variable frequency drive (VFD), you will have a grinder that has the versatility to grind metal at higher speeds and to run slower with small steel wheel sets.
The next best option is a grinder with step pulleys that will give you up to 4 different speeds depending on the pulleys you use. I recommend you use cast iron pulleys vs. aluminum as they have less vibration at higher speeds.
If you use a grinder with a fixed speed RPM, here is a link to the Surface Feet Per Minute you can expect from 4″, 5″ or 6″ drive wheels used with 1800 or 3600 RPM motors.
Wheel Speed SFPM