How do I purchase DLC for Bioware games? Through Origin or Bioware?

I am fairly new to using Origin. I was planning on purchasing the following games:

1. Mass Effect trilogy
2. Mass Effect 3 digital deluxe upgrade edition
3. Dragon Age Ultimate
4. Dragon Age 2.

I know not all the Mass Effect 3 DLC nor the Dragon Age 2 ALC is available there.

My questions are:

1. If I purchase all the games above, can I purchase the DLC from Origin? Or do I go through Bioware?
2. If I buy the extra DLC, does it get installed through the Origin client once I decide to install the base games?
3. If I purchase everything before running the install and play, will the Origin client automatically install the DLC if it has to be bought via Bioware?

While viewing any game with DLC in origin, there will be a letter 'i' which brings up an information popup containing add-ons, which will then display all available DLC. From here you can buy and download it all provided you already own the game.

Note specifically for Mass Effect: Bring Down The Sky, this is not available on Origin and but EA have provided it as a free download. Pinnacle station is available on Origin (although I'd personally give it a skip as it doesn't really add anything).