What ornaments will it set?
Nailheads (like Bedazzler studs)
Preset rhinestones (like Bedazzler rhinestones)
Rhinestone Rivets
Loose stones with rim settings
The press does not handle Tiffany Settings at this time.

Is anything else needed apart from the machine?
Yes. In order to set an ornament it has to be held properly in the setter. This is done by way of a die. You install the die in the press and then insert the ornament into the cup of the die. Some dies are suitable for more than one style of ornament, but it is possible that you may need a unique die for every ornament you want to set.

Can it set anything else.
It can set almost anything provided you have the right die for it.
A custom die making service is available. Price approximately $180 per die.

Is it strong enough to set on heavy materials like leather belts?
The ability to penetrate your heavy suede, leather and nylon materials is not limited by this equipment. The limiting factor in a heavy duty application is the ornaments themselves.
Are the prongs of the ornament stiff enough to penetrate the material without bending?
Is the prong length long enough to fold over on the back side of the material?

You can get some idea about this by first testing by hand if you don't have a machine.
Tests by machine are better because the die offers some support to the prongs during penetration.