I do already have the OSX Leopard disc anyway, so that doesn't matter to me.
As long as you are aware of the short comings of such and have an avenue to install an OS, that is important.
Many people come here after the fact, asking about OS discs, and are chagrin to find that the discs sell for more than the machine did.
That said, if your Leopard disc is not a black, retail disc, well, then you have NO disc. The gray, OEM discs won't work.
Lots of others on eBay seem higher
If like auctions here, they aren't selling, either.
If you consider the price to be acceptable, possibly not too high.
Without discs, with the original, pathetically small hard drive, existing RAM, likely the standard, inconsequential Radeon 9600 Pro graphics card, there is nothing that really sets the machines apart.
Given the age, the history of and the potential failures/problems of the G5, I would try for £100 (or less).
I definitely would go for the 2.3 GHz machine, as, one, it is faster, and faster counts, and, two, it has PCI-X rather than standard PCI that the DP 2.0 GHz machine has.
The Early 2005 2.3 GHz machine has better architecture.
It also has 8 RAM slots, whereas the Early 2005 2.0 GHz machine has 4.
There were two earlier DP 2.0 GHz machines, either with 8 RAM slots, but both were yet underspec'd as compared to the 2.3 GHz machine.
Both had reliability issues as well.
The first 2.0 with the weak 64 MB Radeon Pro and the second with the miserable Geforce FX 5200 were both laughable in the graphics department.
The current RAM is really inconsequential.
I see some good deals on used, G5 RAM on the net.
Even new isn't too expensive.
The DP 2.0 machine is a "run of the mill" machine for the era.
The 2.3 had one of the better reliability ratings:
http://www.macintouch.com/reliability/pmg5.html