older vs newer Vita Mix
I have both types, the old 3600 with the stainless container, and the 5000 with the lexan container. The older models have a different design for the blades in the container. They are dual purpose and can be used for wet blending (smoothies, soups, etc) and to grind grain and knead bread dough. The newer models have separate containers using two sets of blades. One only for wet, the other for dry grinding and kneading. They are both fine machines, with the 5000 series having a slight edge in the smoothie department. The texture is just a little silkier in the newer models. Either will run rings around a run of the mill Oster, Waring or similar blender. The Vita Mix is a wonderful tool, and would make a fine addition to anyone's kitchen. They are powerful, dependable, long lasting machines.
If you are thinking of buying a used model, I recommend you don't buy anything older than a 3600 model. They just aren't as powerful. Also, parts are not available for these models.