I just wanted one PPC Mac also.
I'm moving right now...and this is a start(with a lot more added since and more still to go) 7 years later...
As you can see, I'm partial to G4 towers. I have owned an example of every "G" series Mac save for the iMac G5, and they all have their good points save for the Mini

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The DA/Quicksilver(which are more alike internally than different, despite having very different case designs) are my favorites for building projects. I've also held back(read not put into storage) a dual 1ghz Quicksilver, which is one my my favorite OS 9 computers. The platform offers a lot of potential both for OS 9 use and OS X use, plus has one more PCI slot than earlier models so you can cram an extra expansion card in. That's not a HUGE deal on these towers-the beige towers often had 3 slots and there are often 4 cards I want to install-GPU, ATA or SATA card, USB/FW card, and 10/100 ethernet. The later towers give you a dedicated AGP slot for graphics and also have at least 10/100 if not 10/100/1000 on board, so that's not an issue. My usual builds have a GPU, USB 2.0 card, sometimes a modern "G" Wifi Card(which doesn't work in OS 9, but is great for OS X), and sometimes an ATA or SATA card(this gets over some limitations inherent in the HDD controller on the graphite towers and earlier Quicksilvers) .
The earlier "Sawtooth" towers have their pluses and minuses, but would also be a good choice. The AGP slot is 2x, which limits your GPU options for OS X, but all OS 9 compatible cards will work and not be bottlenecked. One of the big Sawtooth advantages in my book is that PSUs in all of these are aging, and they can be a perpetual achilles heal. An ATX PSU requires only minimal modification to work in a Sawtooth(IIRC, it's swapping two wires), while the later ones need more extensive modifications/jumpers and also lose ADC power if you use that(I do).
If you don't mind the price or the limited upgradeability, the Cube is a fun option and is classic Apple design. Throw a better GPU in it, perhaps a mild CPU upgrade if you care to, pair it with an ADC monitor, and you have a perfect early 2000s Apple set-up that not only looks the part but also works great in OS 9. In fact, I have one here that I use as a gaming machine-it has an 800mhz upgrade, a Geforce 3 GPU(rare and pricey-a Radeon 7000 or Geforce 2MX will get you a lot of the performance for a whole lot less money) and is connected to an ADC CRT.
That's just my thought. iMac G3s are fun and quirky, and are the computer that put Apple back in the right direction. The later slot load models are better all around as users than the earlier tray load ones. They are right at home in OS 9, and can play a lot of games decently, although the GPUs leave something to be desired for some heavier more 3D-intensive games.
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A few other thoughts...
I'm in the "just say no" to classic camp. I've used it some without trouble, but a lot of games seem much happier running natively in OS 9(or earlier).
Also, I'm a BIT hesitant to recommend a portable for OS 9 use these days. The snow/white iBooks are a great value, but will limit you somewhat in terms of GPU. The Pismo is good, but in a bit of the same boat. I love TiBooks and they would be perfect aside from the fact that many are falling apart. The first gen 17" AlBook has emerged as a hackable OS 9 computer, but has a nasty tendency to eat its L3.