A PC power supply might work if the voltages are correct for the motors & laser.
can anybody please check this and tell me my mistakes?
I suggest you build, program, test, and debug one "part" at a time. You've got servos, laser controls, and a motor (and motor control?).
There are two power related issues... You need to power the motors and laser, and you need to control the power unless the motor & laser run continuously. High-power LEDs also need additional electronics if you want to control them with the Arduino. [u]This example[/u] shows you how to control a high-power device using a MOSFET.
The servos just need power and a signal from the Arduino. The electronics to control the servo power are built into the servo.
Laser diodes and high-power LEDs require a special constant-current power supply. And from what I understand, some laser power supplies have an optical feedback circuit to monitor/control light output. Dimming high-power LEDs requires additional circuitry. I don't know anything about dimming lasers, or if it's even possible. (As you may know, you need dimming if you want to get all the colors of the rainbow from an RGB LED.)
If you use a bare laser diode or LEDs, you will probably need a lens, or a lens assembly. A laser module will come with a lens.