1) A BlizzardPPC card is more than capable of playing MP3 without any additional hardware. Although a sound card would give you better sound quality than Paula.
2) A Power Flyer is an IDE interface for the A1200 giving you faster hard drive performance than the A1200's built-in IDE (which is notoriously slow)
3) A Scan doubler doubles the horizontal scan rate of the Amiga's PAL / NTSC screen modes from ~15Khz to ~31Khz, so that they can be displayed on a standard VGA monitor.
4) No, there is currently no faster expansion for the A1200 than the BlizzardPPC. There are some projects that have been 'in progress' for some years, like the SharkPPC, and the PowerVixxen, as well as a couple of Coldfire boards, but nothing has yet been seen and I wouldn't hold my breath for any of them (The ratio of new Amiga hardware announcements vs actual hardware being made available to buy is very low)
5) Yes. any IDE CDROM or DVDROM will work connected to the A1200's internal IDE via a IDE splitter or buffered IDE interface. You just need to install a CDROM filesystem like CacheCDFS.
