you can indeed use the A500 psu, this will provide about 35 watts of power for your A600.
You can indeed modify a PC PSU ( i have done this countless times)
heres a brief (and simple) overview
see easy peasy
if you need anymore help just let me know.
for part 1 you could just use a standard floppy drive connector to the A600 mobo and use the blue -12 (pin12) and place this in the central slot of the A600 power socket (thus completing the circuit
)
you will still need to make a power on button but thats easy