Basically the 4 kits in this Robotics section are quite different.
They range from VERY SIMPLE to QUITE ADVANCED.
However a few notes on how they work, will help you decide which to use. Each kit comes with a mini PC board that has an overlay showing the placement of all components, pre-tinned pads and solder-mask and all components to fit on the board.

THE SOLAR ENGINE

The SOLAR ENGINE is the simplest design but very difficult to get working. It only works with a pager motor and a low-current solar panel. It consists of two transistors and a diode. The motor turns ON when the main electrolytic is charged.
This circuit does not work with a very low-current motor and a high-current solar panel. The motor keeps running all the time. It also does not work for a normal 3v motor - the electrolytic does not have sufficient energy to turn the motor.
Cost of Kit: us$ plus postage

SUN EATER 1A

This circuit is over-designed when compared with Sun Roller MkIII. It requires 3v for operation (6 cells). The circuit suffers from the same problems as the Solar Engine circuit and “just any” components cannot be used for the motor and solar panel.
Cost of Kit: us$ plus postage

SUN ROLLER MkIII

This circuit requires only a few more parts than Sun Eater 1A but will operate on a much wider range of motors and cells.
Don’t let the transformer put you off. It is one of the cleverest components in electronics.
Cost of Kit: us$ plus postage
12v motor: us$ (1 required)
100mA solar cell: us$ (4 required)
200mA solar cell: us$ (4 required)

ROBO ROLLER MkII

This is the ultimate circuit. The inclusion of two flashing LEDs allows the circuit to detect the direction of a light-source. Read the article for the photo-sensitive nature of flashing LEDs.
Cost of Kit: us$ plus postage
12v motor: us$ (2 required)
100mA solar cell: us$ (4 required)
200mA solar cell: us$ (4 required)