As said, any retail disk is fine Leopard was released a little over 2 years after the last PPC Macs shipped.
I have both a 10.5.0 and 10.5.6, and honestly I see little advantage to using either. Software update will still run the 10.5.8 combo update plus some other stuff with either version. Any Leopard install will require two restarts to bring it up to date(I guess that's a symptom of how much you've done it when you know that). Just buy whatever version you can get less expensively.
As an interesting side note, ALL Leopard disks(and installs) are actually universal. "Gray disks"(system restore disks) are all for Intel Macs, but will install a Leopard version that is bootable on either version. I have the gray disks for my late '07 Blackbook, and have actually used TDM to install on a PPC Mac from that disk. Unless you want to defeat the hardware checks, buying a retail disk is usually cheaper than buying a restore disk and a computer to go with it
. As it so happened, I initially bought the Blackbook for the simple reason that I wanted one, and bought the gray disk to get all the factory software(along with the fact that my 10.5.0 disk wouldn't install on it).