I'm such a fan of Defender, I've built one 3 times. Some obvious misses here, first, the controls. The position of the reverse button in relation to the joystick is super important. The MK layout is a mistake. The J Panel should have the artwork on it. The game selection - needs Stargate.
If there is any cabinet that needs a good sound system, it's a Williams cabinet. I hope the sound is stereo at least.