Hey catfish, yes I am from Germany
But I am everything but an engineer, this was my very first approach to work with metal ever.
I start with the last things you mentioned, the heat:
I plan on putting this outside on my balcony (which is pretty well protected already against the weather and only open to the upper front), with a rain/snow/direct sun proof enclosure that will have enough openings (also protected against weather with these
http://www.ventilator-shop.com/zubehor-fur-die-ventilatoren/rohrkrummers/rohrkrummer-90/ and some fine mesh against dust and insects) for enough fans. I will drill as many 108mm holes (with a Starrett holesaw -
http://www.amazon.co.uk/Starrett-Sh0414-Speed-Steel-Holesaw/dp/B0002JT6X6/ref=sr_1_2?s=diy&ie=UTF8&qid=1313349705&sr=1-2 into the enclosure as necessary and mount 120mm fans on the inside, in push pull order (from the back and side-bottom it will push air in, on the front and top front where the gpu's blast their hot air I will mount them to pull air out).
Anyway, the amount of GPU's in my rig will be determined by "how the market goes" in the next months
IF the market does what I want it to do, I will put some money into a powerful watercooling system. This is pretty expensive, as the basics cost about 150 Euro in quality parts + 60-100 Euro per GPU.. But it would cool the system and I could even put it back inside without having a terrible noise pollution but an efficient space heater
(and in the end it will have wheels, so I can just shut them down, pull the plug, move it somewhere else, plug it back and so on)
Power is not a problem here, I have a 16A (3.6kW) fuse available to nearly 100% just for the rig (for whatever reason the smallest room in my apt. with the least power consumption has it's own fuse, where my living room with all the stuff shares the fuse with the floor where I put the vacuum cleaner in o.O). My building is pretty new and has good cabling, though.
In my frame is some torsion, this is simply because I didn't do exact measurements in the beginning with the first bars I put together - if I would have to build another one, I would know better what to do in which order, and I would take my time to make the edges more smooth. But I just wanted to put this thing together
I would also use a mix of wood (for the outer structure that does not come in contact with any hardware) and metal then, because the wood is easier to work with and the metal would be only used for parts that need to carry the computer stuff.
Pre-drilled lengths of metal would be nice to have, anyway
But as soon as I get this one
http://www.proxxon.com/eng/html/27100.php (for my drill stand) accuracy in drilling the holes will not be a problem anymore