Powering HID lamp from 240 volts
A 40W ( 3.5A @ 12V ) HID lamp is equivalent to 70W halogen lamp in terms of light output, so it is not really a replacement for a 300W lamp. As for the SMPS question : any good quality computer power supply rated for 350-400W should easily provide 10A of current at 12V rails ( inrush or continuous )