There are a few requirements and some helpful tips to keep in mind when picking out the hardware for Mac OS9.

Try not to get bitten by the "Mac Bug" (I have personally collected and still own all of the above... pick one only)

Without getting too bogged down in the technical jargon; the computer must be capable of booting to Mac OS 9 directly and without OS X present whatsoever. This is as opposed to running OS 9 via the “Classic Environment” (a hardware and software abstraction layer in Mac OS X that allows OS 9 applications to run on Mac OS X). The main problem with Classic is that it does not allow the direct access to hard drives and audio cards (if you're building an audio or video studio Mac). Retro gaming also benefits greatly from booting directly to OS 9. Tiger was the last to support the Classic Environment and Leopard and Lion need to "shave sheep," so take my advise and spend less money and get some classic hardware for your classic OS; if you are unsure if a model can boot directly to OS 9 then check out all the details at this incredible site:

http://www.everymac.com/

Select a Power Mac G4 tower (or PowerBook G4 laptop). These Power PC units will be the fastest units that can still boot directly to OS 9 and they will have more cache (memory that resides within the CPU that cannot be changed). G4 models with 1 to 2 MB processor cache are twice as fast when processing audio & video as the cheaper G4 eMacs, iBooks and iMacs. Again, be aware that you must check out the machine’s technical specs very carefully to insure you get at least 1 MB cache and that the G4 can boot directly to OS 9 (all Intel CPUs are automatically out of the question). Also, be aware that more expensive dual processor G4s may not be any faster since most OS 9 applications will ignore the second CPU all together. If you follow my guidelines you won’t need a dual processor. I have personally been involved with audio projects that have over 60 tracks of 32 bit audio with effects on a single core G4. By the way, some of the fastest OS 9 Power Mac Towers (Quicksilver G4 933 MHz and MDD 1.25 GHz 2003) can both be purchased for around $200. The MDD (Mirrored Drive Doors) G4 is significantly louder than the Quicksilver G4 and is sometimes called the "Wind Tunnel" G4 tower.

MDD G4 1.25 GHz
Aka MDD FW400
"Holy Grail" of the G4s

Although the eMac G4 1 GHz "Combo Drive" Model, the iBook G3 900 MHz and the iMac G4 800 MHz (17" Flat Panel) all can boot directly to Mac OS 9, they lack CPU cache and are NOT the best choices for Audio & Video applications.