I am a newbie at this, too, but this is how I see things. If someone more experienced disagrees with any of this, listen to them and not me...
If you are looking at a surface grinder that has been sitting for years, do not just start the spindle. Turn it by hand and check for smoothness. Keep trying that for a while to see if it smooths out. If it is crunchy, walk away as fast as you can unless you want to rebuild the spindle, which can cost as much as the used machine or likely more. If it turns freely and smoothly by hand, turn it on and pay very close attention to how is sounds and if there is any vibration. Vibration might be from an out of balance wheel, but might be a bad spindle. ($$$$) Remove the wheel and try again.
A manual surface grinder is a pretty simple machine. It needs to have a good spindle, smooth and with very little runout. The lead screws should have little backlash or wear and operate smoothly. The table commonly just lifts off the machine, so do it and look at the ways and get a feel for what kind of care or abuse it has seen. If it is loaded up with grit and grime under the table, and the scraping and/or flaking on the ways is worn out, take notice. Look for things that have not been properly lubricated and for missing dust guards. Grit and lack of oil kills surface grinders. If it has ball or roller ways, rub the oil off the balls or rollers and the table and see if there are dings in them from transporting it with the table still mounted, bouncing up and down. One road trip like that can ruin a grinder or require major repair$.
The most common stone size for home shop and tool room use is 7" diameter with a 1-1/4" center hole, in various widths. They are the easiest to come by, new or used, have the most options, and cost the least.
Coolant produces better finishes, more accurate work, and helps to keep the dust down. It is also a PITA, makes a mess, corrodes machine and tooling, and needs to be cleaned up after every use. I watch shadonHKW on YouTube. Stan uses mist coolant almost all the time, and seems to do well with it. Still needs cleanup after use, makes less of a mess, does a pretty good job at helping to keep the work cool. Dry grinding leaves a dusty mess, a shop vac hooked up to the wheel guard helps to keep the dust down, but does not keep the work cool or the wheel clean.
There are surface grinders with plain ways, with roller ways, and with ball bearing ways. Ball and roller ways take less muscle to traverse the table. If you have power feeds that work, plain ways are less of a disadvantage.
Surface grinders with power feeds are much more complicated than manual machines. Generally hydraulically driven, they can leak and/or fail. When the hydraulics are not working, it is usually a bear to operate the table manually.
Magnetic chucks are not needed for certain classes of work. There are other ways of holding things, like vises and various fixtures. Most people use a magnetic chuck. Closely spaced poles are better for smaller work. The lines of flux from the poles are closer together and do not extend as far above the surface of the chuck. For large work, wide pole spacing is more effective. That is the simple story, but it gets more complicated. Some people like permanent magnet chucks and others like electromagnetic chucks. In good condition, both work. There is lots more to mag chucks than I really know about. Be careful, there is a lot of used junk out there at high prices.
Surface grinders need to run smoothly, and three phase motors run more smoothly than single phase. A VFD gives you the best of both worlds, plus control of the spindle speed.
If you cannot run the spindle of a candidate machine, or take the table off and look at things, or test grind a chunk of metal, you have an unknown, and should not pay more than scrap metal price for it, and expect to have to do a lot of work to it to make it correct.
Just my opinions, and I am a newbie, but I am just finishing bringing an old power feed surface grinder back to life. I did not necessarily follow all of my own advice. It helps to also be a good judge of people, and I judged correctly. 8^).