Like most have said here, the actual fitting of an internal CD-ROM is pretty easy in an A2000 as you have a nice 5.25" bay. Make sure you get a SCSI cable that is long enough to reach over to the bays comfortably, especially if your HDD is also mounted in that area. If the HDD is going to be attached to the SCSI card then you want to make sure that the cable has a long gap in between the connectors as it can be quite a stretch to reach the 5.25" bay if the first connection is made near the card.
Make sure you have at least WB2.x as anything earlier is going to make it a bit of an ordeal.
I hear that some CD drives can be a bit temperamental with the Amiga but I will defer to someone with more experience to list the ideal brand choices.
Have fun with your endeavour.
