LED's got it's advantages... but the cons out-weigh the pros....And the only bad thing I can think of for HID's is heat. I know someone will probably say, "How about your power bill?", but these lights will do more than pay for themselves. You can double, maybe even triple your initial...
This whole LED versus HID argument has been going on since LED's were first implemented into growing. Whatever you use is up to you, but HID is better IMO...Bottom line, it has been proven, time after time.
I have yet to see massive donkey dicks grown from LED's...So my vote goes to HID's...
But if the price of LED's were to drop substantially, they'll be movin' much higher on my list.
You'd probably be better off drilling a new hole for ventilation...And the idea of wiring the metal halide to that socket is a good idea. I just hope you know what you're doing if you decide to give it a shot.
First thing you should do is to figure out the capacity of your breaker box...Then find out which breakers power which sockets and lights.
Then you should calculate how many amps it's gonna take to run ALL your equipment...And determine what you need to do from there, maybe add another breaker...
It's probably better if you just have them in the darkness until you can get that bulb...I wouldn't worry though, a single lights out incident isn't nearly enough to cause any damage! :D GL and Happy Growin'!