Well, adding CC can slow your game, depending on:
1. How much you download.
2. How big the files are
3. How good your computer (RAM size, video card quality, etc.) is.
Now, for making an account, it means like any account, like gaia, for example. And you don't need the proof of purchase: Remember how you have to type that big number when you installed the game ( XXXXX-XXXXXX-XXXXX-XXXX )? You use the code from each EP you have, which shows up on your MySim page.
No, it's not needed to download.
And the downloads shouldn't cause any trouble in your game, unless they need an EP you don't have.
Hope that helps!