heatercores are insanely good, one of the best availible radiators; I'm running a fedco 2-302 (like many many other people), and they can comfortably handle OCed CPUs and GPUs. it cost me 22.50 total (including modding it, building a shroud, etc)
MCW600x blocks don't really need much flow, so a weak-arse pump would be adequate, provided it didn't break. Theres also some other crappy pumps (elec goldmine has a... curious one) that are DC
well durr air is easier
but yes, if done with careful ebaying, it could definitely work. Personally, I just spent 300 bucks on watercooling, so yes, its very very expensive...