Blender is the most powerful free modelling and animation application (with 3dgs support via plug-in)
Wings3d is free and easy to learn
MED is already installed and not that bad as most user say (often such users did not use MED at all nor did read the manual)

FragMotion is a very cheap animation software with 3dgs support

When it comes to the big ones:
For commercial use of the most powerful tools (modelling, animation and rendering) there is nothing more affordable than Lightwave (500 for side-grade, 700 full), Max and Maya are too expensive. Modo could become a good solution in the future with new capabilities for rendering and animating.

Big useful tools (200-500):
ZBrush for creating normal-maps and painting in 3d
DeepPaint3d or BodyPaint3d for painting in 3d

What will be best to teach?
That depends. When you have the time then you could teach Blender. If your customers and teachers do not have the passion and persistence then switch to Wings3d or MED.