OS 9 being old tech and ethernet printing being somewhat newer tech you are probably better off getting an older printer. The trick will be reliable access to the printer. Normally I'd recommend hooking the printer directly to one of the macs (to eliminate the need for an ethernet add on for the printer) but I don't know how well print sharing works between OS 9 and OS X. You'd be using appletalk at the very least and that means you're restricted on how far up with OSX you can go. (I think)
You may have an easier time sharing the printer on an OS 9 machine...
