In it's stock config, the 2300c uses an IDE hard disk - I've put a 10GB in mine. Since older model 68K/Duo owners often upgraded to a 2300c motherboard, there exists a connector on the board to plug the old SCSI hard disk in - quite a nice inclusion!
Keep in mind if you upgrade the HD in a 2300c the mounting holes are out, so you will need to drill new ones to properly fit the new hard disk. At least that's what I found. Oh and take care pulling off the top case - make sure you don't rip off the trackpad ribbon.
JB