OK, I added some stuff and moved some text around -- then I saved this file, because "how to buy a little surface grinder" is actually a FAQ.
I have learned all of this by experience. None of what follows is from conjecture or from something I read. - GWE
The biggest deal buying used is spindle bearings. These are super precision bearings that cost hundreds and hundreds of dollars. They gotta be *perfect*.
Also very important is it should have the original 3-phase motor. If the motor's been replaced by e.g. a Dayton or Marathon, someone's trying to rip you off. Precision grinders need motors with precisely balanced armatures, period. And you don't buy those from Grainger or Harbor Freight for cheap, either.
It should also have all shields for the wheel and table.
It's nice if it still has the stops to limit travel.
Electric chucks are better than manual.
Ball bearing ways are better than slideways.
Hydraulic feeds are real real nice.
Flood coolant is real nice too but messy.
Buy from a manufacturer that is still in business. I like K.O. Lee. A good friend bought new from Chevalier, he's happy too.
It's real handy if the grinder uses standard wheel holders like those made by Sopko. A nice plus is if it comes with one or two Sopko wheel wrenches and real big plus if it comes with a balancing spindle and balancing ways -- little surface grinders do a lot better with a nicely balanced wheel.
One more thing, more of a user issue really. When you're using a small surface grinder be aware that your max. depth of cut is usually .0005" (half a thou) unless you are only feeding in a little bit. Also, go ahead and dress with a diamond to true the wheel but make sure you also have a dressing stone and give the just-trued surface a quick wipe, lingering a tad on the corners. And if you notice the wheel beginning to cut differently, STOP -- it's probably loading up. What can easily happen is it loads up, cuts warmer, more friction less cutting, it warms up from the friction MAKING THE WHEEL A LITTLE BIGGER -- I'm sure you can see this is a classical positive feedback situation. What happens is bad, no matter what it is. Hopefully you don't explode a wheel, and if you do, that's why it needs the shielding.