First off - throw out that notion of cheap right now. Its not going to happen. I set up my frag tank "cheap" and it still ran me about $450-500 (which yes, by many standards is cheap, but a lot more than I was anticipating, especially given the whole college student paying tuition, car insurance, food, etc...). Now that that is out of the way:
1. What/where is the cheapest way/place to get a 400 watt metal halide pendant?
Searching the selling forums likely, especially if you can find a local forum to find things. I snagged a 250W setup (bulb, socket, ballast) for $50, and a 4 bulb ODNO setup for $24 locally. You're best not to rush things. Just post a Want To Buy in the WTB forum, and see what you can find.
2. What is the cheapest frag tank that will be just big enough to be illuminated by one 400 watt mh?
I have the 250W bulb over a 36x18 tank. I did this to provide me with lower light areas on the sides, and fairly intense light in the middle for certain SPS corals. I think with the proper reflector you could successfully light a 24"x24" tank, though if you are going cheap, skip the tanks, and go right for rubbermaid tanks. You can either use the rubbermaid poly stock tanks which are nice, rigid, and fairly cheap ($65 for a 100gal tank) or you can get a normal rubbermaid like container for cheaper and brace it from bowing. That was my intent, but I found the 30 breeders cheaper than those. I think that a 400W bulb would light a 50 or 100gal poly stock tank (nearly the same dimensions, the 100 is just taller) just fine, and leave some lower light areas on the ends for some lower light SPS or other corals. It all depends on how high you mount it.
3. What is a decent, quiet external return pump that would be able to provide enough flow for sps?
Thats a mistake. Dont provide any major component of your flow with the return pump - theres bound to be issues and it decreases the efficiency of a fuge or skimmer. You're best bet is to find a cheap smaller pump for the return and then a bigger one for a closed loop. I've looked briefly into air lifts and might try that in the near future just to see how effective they can be but for the time being a few powerheads were cheaper than the air pump necessary.
As rustybucket145 says, dont go "cheap" on your equipment. Get things that will last or you will end up paying for it in the future. But you definately can get by cheaper than buying things brand new, which is where I saved the majority of my money. I did everything myself, from building the stand do doing the electrical. Speaking of electrical, its little things like that which will add into the cost - things that you dont immediately think about when setting things like this up. I ran two dedicated 15A circuits for the system, just to be safe.
Good luck and keep us posted on how things turn out.
