They can be had for a lot less in the UK but geography does play a big issue in affordability. Sawtooths and GE are fairly versatile in terms of OS support and don't seem to be plagued by problems that their successors had. I would go for one of those and then look at something quirky if funds and space allow.
Just thought I'd mention something on this.
I maintain a PowerMac G4 a work that operates a home-built NMR spectrometer. It needs a PCI slot for the interface card(fortunately a somewhat generic National Instruments brand card with a 60 pin ribbon connector on it-they show up on Ebay enough that we have a stockpile of him) and AGP graphics are desirable to make it easy to run a 1920x1080 monitor(not a requirement, and we could do it with a Radeon 9200, but I'd rather not part with one). It doesn't NEED a G4, but the newest hardware within reason is desirable(it was running on a B&W for a long, long time). Also, the guy who designed it said 9.1 as a maximum, but we haven't had any issues on 9.2.2.
In any case, a while back we switched over to a 466mhz DA, and it more or less worked alright. The PSU died in the DA, and as a quick fix I grabbed a 733mhz QS that was sitting in the room. Interestingly enough, it quit working. We could hook up a scope and see the spectrometer generating a signal(called a "pulse") when the computer told it to. We could also see the returned signal from the probe(called an FID-free induction decay) but the computer wouldn't see it. Putting the 466mhz card fro the DA got it working again-my working theory is that at too high of a clock speed, the software "times out" on looking for a signal before it's actually had time to get back to the computer.
In any case, the Quicksilver PSU died, and I ended up bringing in a Sawtooth. Surprisingly enough, it actually cleared up some software glitches that they had been working around(and I wasn't aware of) but that hadn't been present when using the B&W. Since we've already made an(IMO) reasonable inference that the software is fairly dependent on clock speed, I also suspect that the 133mhz FSB of the DA/QS might have also been causing issues that were resolved by going back to 100mhz.
In any case, it's now been ticking away without a hiccup for a year or better on the Sawtooth...and if we have another PSU failure it's reasonably simple to fit an ATX PSU to a Sawtooth(at least as compared to the later computers).