If you can get down to 10.4.11 (Tiger), you'll find you can run System 9.2 in "compatibility mode" through System Preferences. Mac minis through 2007 will allow that.
Slot loading iMacs from the early 2000's will go up to 10.4.11, and are perfectly happy running System 9.
Both options should be pretty cheap now days, if you can find the units for sale.
You can also run older systems
(7.5.3 etc) on a rooted nook reader.
Although I've never used Tiger on an Intel Mac, I'm pretty sure that it doesn't have Classic mode.
The reason is that it's not actually an emulator, but rather a virtual machine with OS 9 running in it. It talks directly to the processor, and so will only work on a PowerPC processor.
I've never liked classic mode. IMO, for OS 9 use, you're better off with a computer that can run it natively. The best desktop is the Powermac G4 MDD(mirrored drive door) with a 1.25ghz processor. Both single processor and dual processor variations of this are available, although you really don't see any benefit in OS 9 from multiple processors unless you're running a piece of software that can use both(Photoshop 7 is the stereotypical example).
The best laptop for OS 9 use is the 1ghz Titanium Powerbook.
Of course, if you need to connect to legacy peripherals, you need to go a little bit further back. If you need a serial port, you can install one in any G4 using a Stealth Serial Port, although these are a bit difficult to find(the Stealth Port replaces the modem). If you need SCSI, a SCSI card will do the job, but you need to check for OS 9 compatibility(some G3s and G4s shipped with SCSI cards that will work natively). If you need ADB, a B&W G3 is newest computer with such a port(these can be upgraded to a G4 pretty easily, although a G4 offers little benefit unless you're running Altivec-enabled software).
System 7.1 was the first that would run on a PowerPC processor, and only a handful of them at that. System 7.5.5 is better in pretty much every way. OS 8(note the change in naming) is a big mess, and you're better of skipping it for OS 8.5. OS 8.6 is a free update(it's still up on Apple's site) for OS 8.5, and adds support for USB mass storage devices. Pre-G3 computer officially stop at OS 9.1, although 9.2.2 runs great on them(installing needs a little bit of work).
If you want to run System 7 or earlier, you need a 68K processor. Better 68K processors(I think the 68030 and 68040) can go up to OS 8.1, but for your own sanity stick to OS 7.5.5 with them.
As for older versions of OS X-any G3-based system can run up through OS X Tiger(10.4.11), although OWR systems(anything without built in USB) requires using a program called XPostfacto to install. NWR systems that lack built-in Firewire need XPostfacto to install 10.4.11. Any G4 system can run Leopard(10.5). It's straight forward on a system with AGP graphics(you just need to bypass the processor speed check on systems with processors slower than 867mhz-there are several ways to do this) although those without AGP graphics need a fair bit more work.
I have a computer set up to boot every version of OS X from Public Beta to 10.5. Honestly, there's little reason to actually use anything below 10.4, as it loses no functionality over earlier versions and software availability is much better(including an up to date web browser).