I plan on getting the DLC it's only just a matter of when i get it. Was possibly going to wait til there was a price drop or possible deals for them. That said, should I beat the main quest before getting them or playing through them? Also would I have access to all the new features in the base game if I got the DLC.

The DLC is generally geared towards higher level characters, but completing the MQ isn't necessary for any of them. Some of them might require you start the MQ, but they're separate stories. If the DLC is installed, then all the new features will be there.

Choice is totally up, you actually get different dialogue depending on whether you have done the main quest or not (and some side quests) but that's about it, you still get all the same loot and stuff regardless of if you have done quests..

I would wait, especially with Dawnguard, unless you plan to finish it right away. The reason being

Unless, of course, that doesn't bother you.