when is the best time to download addtional content? after completing the main game or before?

FK

Before since it adds perks and each one raises your level cap by 5.

Before, obviously. Besides aforementioned level cap raise the DLCs can be done when you feel like it. You can play through the entire game without paying any attention to the DLC.

Indeed, before, because it adds new perks, and OWB adds http://fallout.wikia.com/wiki/Logan%27s_Loophole if your worried about leveling too high.

Edit: Wrong link