'Early to late nineties' are not really that old machines, so you should not have that much of a problem as far as memory, or parts come to that, if you are looking at 90's machines, especially if it is a big name that is still around, and even for 70's~80's machines, brands like Mitsubishi will offer rebuild or repair parts, if the original is not available.
You may want to stay clear of control makes that are not around anymore, if one of these becomes a 'basket' case then your only option would be to retro-fit if you cannot locate parts.
You should be OK with the likes of Mitsubishi, Fanuc, Fagor, Siemens, Heidenhain, Mazak's are nice machines used mostly Mitsubishi control , some were Fanuc.
Some of the older Cinncinatti's parts are getting scarce, but the machines themselves were built to last.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.