Ahh... The picture helps a lot! I had misunderstood, thought you were pumping water through the pipes instead of using them as radiator surfaces. What you are doing should work well as a cooler, but I'd still worry about contamination to some degree.
The primary problem with contamination would be clogging up the waterblock, especially if you are using one that relies on jet impingement type operation. A secondary issue would be any corrosion problem caused by dissolved contaminants. (BTW, I don't know how much good it will do you to use distilled water in that setup, IMHO you're going to get enough crud in the water from dust, etc. to make it kind of a moot point....)
It would be difficult to get a good filter on the intake that wasn't over restrictive. Best I would suggest is to position your intake so it is at midlevel in the tank - most crud will either float or sink, so sucking out of the middle should help avoid some of it. Secondly try to seal up the top of the tank as much as possible, including the insides of the pipes.
Radiator coolant is reasonably good as an anticorrosive, but it isn't real good as a biocide (the temp changes in an automotive cooling system are what keeps the wildlife under control) so you will probably need additional biocides. Airspirit over on Pro-Cooling speaks highly of non-chlorine / non bromine hot tub and spa sanitizer. You might also try some of the anti-algae stuff you can get in aquarium stores.
Building Dual Athlon MP system, Tyan mobo, U160 drives, Server Cube case, Linux ONLY, lots of other goodies. Will water cool, attempting to make as silent as possible.