This is interesting. All the Power Mac G5s I have picked up have been $40 or under, except my Dual Core G5 which was $60. All the $40 ones needed work, however some were Liquid Cooled and others were air cooled. You just need to hunt around and see.
The Quad Core (Dual Pump) Delphi systems and Panasonic pumps were much, much more reliable then the Delphi models. For power, I'd go for one of these. These LCS might need a small service; flush the system, liquid refill, o-ring replacement, thermal paste replacement, etc, but leaks have not been heard of on these models.
The Panasonic system can be found in the Single Core, Dual Processor models only meaning DP 2.5s and DP 2.7s. The inlet frame for the processor is much different then the one for the Delphis. The inlet frames for the Delphis are about 1/2 and inch to an inch thick. The Panasonics inlets are about 3-4 inches thick. The dual pump systems were only made by Delphi and only on the Quad Core models.