Hey there AmigaClassicRule.
The internal IDE in the A1200 is a bit tricky since many factors are used, accelerator card, scsi.device driver, drive etc but the "max speed" would be around 2,5MB/s but expect 1-2MB/s which if I have counted correctly will be enough for the kind of bitrates you will watch on your Amiga 1200.
But if you want to upgrade you could get your hands on a accelerator card with SCSI and if that fails the FastATA controllers are a good option.
Tho if I where you I would go for the Ide FixExpress instead, it's cheep and double the speed (atleast) of the internal IDE in the A1200.
So you have a few options to chose from, but first find the quickest accelerator card you can get if your planning on watching movies, then decide what road you want to go with the hard drive controllers.
I could be wrong with my bitrate calculations but I think that you will be all right with a accelerator card and the internal IDE.
I am not a video buff so video is not my strongest area of knowledge.
Edit: Well it's 05:54am right now and im quite tired, so ill run along to bed before my wife wakes up
ill get back to you when I wake up