What's a centrifugal fan? You can build a closed air reflector with same parts adding some material for a frame and glass..
600watts is pretty hot... but where there's a will there's a way! you gotta have a little MacGyver in you to grow on a budget! I personally wouldn't be able to let an HPS light sit unused, but I completely understand the need to not spend anymore money.
The LED will be fine... you'll grow nice buds, you wont know what you're missing anyway. Keep the 600 watt HPS... if you keep growing, you'll find a way to make it work next grow or two.