I've already reseached the RD pumps, in particular the RD-20 you refer to.
Read
this document if you haven't already.
For 24V operation you could just run a step-up transformer attached to your PSU's 12V rail to bump your 12V rail to 24V for the pump. Such transformers around about $20US (I live in Australia so you can probably get them for cheaper in the USA).
However, we get to the serious problems. They suck down a fair amount of power, being around 60W (2.5A @ 24V under normal use). That's a lot. Worse is the startup in-rush current. 8A @ 24V, or if you were running a transformer, probably around 18-20A @ 12V in-rush into the transformer. That's going to cause most any PC PSU I can think of to overload and shut-down before the PC even powers up the moment you push the power-on button. My Antec 550W PSU supplies 24A on the 12V, but given the in-rush load of other peripherals and the motherboard, there's no way even it could handle the power-on current requirements.
Then we get to the price. Find out how much one costs, and then get back to us. Word of warning: make sure you're lying flat on a soft surface before the representative gives the price to you. When Iwaki says "easy on the budget", they're referring to company budgets, not an individual's.
As for the Iwaki's noise, "
as low as 40dBA" is not really heartening reading material on an advertising brochure, if your quest is silence.
