Hi. I see you are in M'sia so the way I use billboard MH system can be copied by you on a very very low price (which can get to around $40-50 right here). If you are going to use such system, make sure you do a research in the availability of high Kelvin MH bulb (which happens to be 150W as the most favorite size for billboards here). Should you find 150W to be easily available, stick with it. I suggest using 150W Osram 5500K. Do not worry about it is stated as a 5500K, I see that this bulb is too white to be called a 5500K, more like a 6500+ K fluoroscent to me. It is darn cheap also at a mere $10 or so (It must have been Osram licensed Indonesian or China product). As far as seeing how it burns, it looks almost the same as the aquarium counterpart.
Does it works? yes. I installed a CO2 injection yesterday on my 260 gal tank lit by 3 x 150 billboard MH and plants started to pearl within an hour (they usually did this only during water change). This is a proof that such MH effective in kicking plants into photosynthesis. Just be sure to pick a fixture that fits to your tank in terms of light coverage (look at the reflector and see how wide it beams... you have to adjust the height from water if it beams too wide/narrow)
1. Does the outdoor unit MH suitable for indoor use ?
So far I say yes. With 6 hours a day of full 3 x 150W blast along with 4 x 40W fluoroscent, the fixture remains steady burning and output stays constant. I have been using it for almost a month, and a well known shop I know had it running for almost a year without any failure. It does not melt or toast. Just be careful to light it in a considerable duration at once and give it a descent air circulation. If it stands the heat of day, cool of night and blistering sun when its hot, acid rain, pollution , etc, it should be able to survive indoor.
2. Would the electromagnetic wave goes interferring with my electronics or even harmful to human ?
I dont know about this. Older type ballast uses rolled wire in the system which utilizes magnet and electromagnet wave, and it seems no one have problem with that. I have my MH running within 4m from my wireless telephone, 5m from my 2M communication radio system, 3, from my TV and I have my mobile phone in my pocket when I did my tank. No interference so far. My mobile phone actually is a more electromagnetic generator, my monitor flickers whenever I'm about to receive SMS or a call.
3. What are those possible bad factors to be considered?
Aquarium supply shops hate you for being such a cheapskate
Well it is clear enough safety is the concern here. At just $40-50 a set, you have to be careful of what you choose. Be safe and pick the best billboard system you can find and make sure you did a very save routing for the wiring and stuff. I mean this is a DIY project and it is demanding in terms of care when setting up.
4. What is the different between single-ended and double-ended MH bulb in terms of advantages ?
I dunno about this.... maybe someone knowledgeable can explain it. So far I have seen only the type of MH bulb that has a fitting on both sides of the bulb (guess it is the double ended MH bulb). Advantages : so far I have seen only the double edged one, so definately better availability and lower prices.