All info you could ask for about Amiga hardware is at
http://www.amiga-hardware.com/.
There is also a SCSI solution which sits on top of your internal IDE interface, but I wouldn't recommend that.
Your internal IDE controller will run an IDE CD-ROM just fine. But there is no external IDE standard, so you'd either have to have it outside, with external power and an ugly ribbon cable protruding out of your A1200, or build it all into a tower.
