Hi, pangabell -
To summarize -
The Intel-processor Macs are not supported by Apple for OS 9 at all, either for booting or as Classic.
No G5 Mac can boot to OS 9, although they can use it as Classic.
If you need a machine which can boot to OS 9, you will need to get one of the following -
An OS 9-bootable G4 tower. These include those G4 MDD (Mirrored Drive Door) models whose processor speeds are 1.25GHz or slower, and which do not have a Firewire 800 port; or any earlier G4 model.
An iMac that is an original G4 model, or any G3 model.
Any iBook G3 model (none of the iBook G4 models are OS 9 bootable).
A PowerBook G4 which is no newer than an '02 model.
An eMac which did not come with a SuperDrive nor USB 2.0.
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This Apple KBase article shows which machines and models are supported by Apple for OS 9 booting -
Article #25114 - Mac OS 8, 9: Compatibility With Macs
This Apple KBase article clarifies some of those -
Article #86209 - Some Computers Only Start Up in Mac OS X
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Judicious shopping on places like eBay can produce a good deal on an older Mac.
For higher prices, but with better warranties and often recourse, some on-line sites offer used Macs. A sampling of possibilities -
Mac Solutions
All Mac
Hardcore Mac - this one is an Apple reseller
Wegener Media