Oh geez. I didnt even read the whole thread. In a former life I ran a 10,000 sq ft data centre, and build cluster computers for many different outfits.
Luckily we had real budgets so we bought AC's and proper server rooms.
Cheaper solution is to use environmental cooling. see intel's site for more info on that.
You can move regular temperature air in, just gotta move more faster (exponentially) as the temp diff between outside air and your cards is lower (course ,the cards
will just get hotter if you arent cooling them enough, then fail or otherwise). Obviously having them throttle down based on operating temp will keep your investment safe.
Try to keep your exhaust air from incoming air as seperate as possible - duct away from the cards as you can. Beware however, if there's a failure of incoming air for any reason, your ducting may insulate your cards from new cool air, and you'll quickly achieve burnout (or possibly fire)? One way to do this is to keep two air moving systems on the same circuit as the rig itself, so if the power goes out then everything is off, not just the AC/air mover. You dont want your gear in a small box with a tiny air buffer that it can heat up to very hot in a short time while still powered. Thermal trips on the mobos will also help of course, they can just shut off the whole system if too hot.
Ducting upwards is a good way to keep exhaust from one row away from the next. (course you need to get it back away from the shelf as per the diagrams first, so back, then up), so you need incoming air coming from the 3rd dimention, from the side, then thru the back, then up and out, then there's no mixing of air, or exhaust air becoming input air to the next row. (You could also have air coming in the bottom, but now you're talking raised floor).
Im thinking at approx 0.7-0.9W/W to cool with a professional chiller, that it isnt worth it. Cheaper to throttle on hot days and use ambient air. 1.8x the cost of power is a big increase in cost.
a 1 ton chiller will cool 12000 BTU/hr, or about 3500W of cooling ish. That will cool 4000-5000 W of dissipated heat. a 1 ton chiller is about $5000 USD new in places, i see some for $3k used. Install is going to be at least that (then count shipping). http://www.dimplexthermal.com/UsedChillerStore.aspx
15 ton for $18.5 K, add in $5-8k for ship/install assuming you have some power busses in place already for 3phase etc. Pushing $25K gives you cooling using ~50kva for 60-75,000VA (depending on many factors). 50kva is about $6.25/hr in my market (assuming you're not enjoying industrial rates, in which case it's about 60-90% of that), which is ~$30/day or ~~$1k/mo. not huge, the capital cost and install will beat operating in amortization for 24 months or more.
However, I think bigger blowers will give you more ambient environmental air, just gotta clean your gear more often. Itll turn black if you're in a city/near any roads due to diesel particulate. Pretty gross.
Filtering the air just means lower airflow due to backpressure/drag. Cleaning is cheaper
Doesnt affect operation for the most part, unless your fans are getting gummed up. Watch for humidity issues however in very dry climates/winter, could get static and sparks and dead gear. Wetness wont be an issue if the room is always 20F > outside... however, gear may really suffer when its 90F out and 110-130F inside. So will the humans.