I grind with a Farmall M. I like using that one because it is so maneuverable around bins and feeders. It does just fine with everything except alfalfa bales. It doesn't like those but it grinds them just fine. My M is my loader tractor and has loaded tires - it is stout. Growing up we used a Farmall H which was fine but that isn't a very big tractor (we had sugar sand and didn't battle mud). When you have to drag a loaded grinder through the hog pen after it has rained 3 inches you are going to want some size behind it.
I have a JD 400 that I really like. They say to stay away from the 700's. Mine has the side auger which is a must as far as I am concerned. I don't have scales or hydraulic anything and do fine without. I don't use the grinder every day so I am good with driven augers and such. I know my mix ratios so I don't have a use for scales. I'm not that exact with my hog feed.
I have heard that the gearbox on them (400's) is obsolete but I see parts on eBay. I haven't needed anything but bearings and those I bought generics and had no problem finding them. If the seals go in my box I am going to fill it with grease and keep grinding.
You get what you pay for. Look at resale and that will tell you quality of the machine. I would either go with JD, New Holland, or Gehl. Look it over close. If it has ever been stored outside run from it as fast as you can. Like I say, I wouldn't do without the side auger. The rest is all extras. Look at the hammers, too. They can usually be turned three times. If they have been turned to the last side you had better bet the whole works is about worn out. Mine had been turned once and I have left them there.