there should be a tag right by the back of the computer right were you plu it in. The tag should have a bar code with a serial number on it, below that it should tell you in one line the megaherz of the processor, the size of the hard drive and the amount of memory it has. This is a good indicator of what it has, unless it's been altered along the line. If you did have the keyboard you could boot it up then go to the system profiler to see what is currently installed for hardware on your computer. My guess is that it's probably a 400mhz computer with a 20gb hadrdrive and 128mb of ram. Depending on what's installed for hardware and software you may get anywhere from $200 to 500.
Check this place out they may be able to help with the keyboard and mouse,go for the optical Pro mouse and Pro keyboard (it's a little bigger than the standard mac keyboard)
Hope this helps
Joe