I have an external SCSI CD-RW drive that I paid $2 for on ebay, it's black, was made for some Mac line but works fine on my GVP controller. Honestly though, with as big a tank as the A2000 is I'd recommend you purchase an internal drive rather than external!
Also as others have said above, the A2000 has a huge range of expansion options available for it. You could probably pick up an accelerator+RAM+fast SCSI controller for a very reasonable price on ebay or amibay, depending on your budget, that's what I'd recommend. Also get an Indivision ECS if you'd like to hook up to modern LCD monitors or TV's, and expand your chip RAM if you plan on working on anims, etc. Good luck!!
