The retail version is Steam; most of the data is on the disk, so it is just the updates since the DVDs were produced that needs to be downloaded (voicework, etc).
With both the retail and Steam direct version, the game will not require the Steam client to play (unless you want to use the Steam workshop for mods, or invite a Steam friend through the Steam overlay).
Once you register the game on your Steam account, I don't think there should be any problem getting the DLC.
The kickstarter said the soundtrack would be available in several formats, including lossless. Since kickstarter backers can choose to get the game from Steam, I would think the soundtrack in the DLC would be the same.
