Toaster works fine under 3.1. You shouldn't see any compatibility problems.
4MB and 030 is just barely enough for 3.9. With the Toaster, though, you're probably better off staying with 3.1. 4MB is barely enough for the Toaster.
Those SCSI adapters are for connecting 50pin devices to a 68pin cable. As far as I know, no peripheral for the 2000 uses 68pin connectors, so you probably don't want those. I bought some from AmigaKit (for much less) for my CSPPC but haven't installed them yet so I can't speak to their effectiveness.
