The A1200's built in IDE interface runs at PIO 0, it's ####-slow.
You will be able to use your SCSI-3 disk with a blizzard SCSI kit, but you'll need an adaptor to convert it down to SCSI-2 (I think).
Anyhow, I wish you luck trying to find a blizzard SCSI kit for a reasonable price, last offer I was given was £50 from some dude off amibench, so I bought a PowerFlyer Gold for £20. But I'm still looking for a reasonably priced blizzard SCSI kit, so you'll have some competition when looking for one. :-D