Cards are no longer manufactured so you will only find them second hand (and generally in the places already mentioned, but have a look at Amigakit.com as well).
If by optical drives you mean the LS-120 types, don`t bother. I had one for a while, it was a bit dodgy and I swapped it for a CD-RW and never looked back. A hard drive on the other hand is almost essential.
SCSI sounds like it`s faster and more stable than IDE, but it`s more expensive, more rare and more difficult to set up.
The port in the A1200 is indeed an IDE port. You can attatch a 2.5" hard drive to it without any other modification and configure it from workbench. I believe you can use 2 devices with a 2-way cable with no other modification req`d.
IDE-splitters, (or "Buffered Interfaces") allow you to attatch up to 4 IDE devices, and require additional software (e.g. IDE-Fix 97 or EIDE 99 which normally come with the interface) They come in various shapes, forms and prices, and allow you to attatch hard drives, CD-Roms and Writers, Compact Flash readers, etc.
Cheers