Most likely any surround system you buy will be able to decode Dolby Digital and Dts surround sound, but double check it for the specific system you buy.
A system that has full HDMI inputs (not just HDMI passthrough) will allow you to play the lossless audio tracks on the bluray discs, but that’s not a critical feature as the bluray discs also have Dolby Digital or Dts surround on them as well.
If the system you buy doesn’t have HDMI inputs, make sure it has at least an optical audio input (provided your bluray player has an optical output), so you can experience the best quality sound.

Not directly. You will need the speakers and some type of surround amplifier/receiver … not just the speakers. That said, all Blu-ray players have built in 5.1 dolby digital decoders (and many have advanced decoders for 7.1 formats increasingly used on Blu-ray (but not DVD) disks).