As the title say's I'm new to the game and I'm wondering when I should play Varnholds lot. Should I play the main game to completion first or just partway through. or should I play the DLC first?

After troll trouble is the best bet. You will be somewhat accustomed to the systems in place by then and wont miss content associated with DLC. That being said, Varnhold DLC is quite a but above the curve then it comes to difficulty, so be prepared for that.

I know its ment to weave into the main storyline so if my choices in one affect the other i want to play them and get to see the consequences play out

Whatever happens in the DLC will affect you game a bit, yes. You will be asked to import your endgame state from the dlc shortly after troll trouble. The consequences are not that dramatic, but they are there.

My advice if doing it right after you finish troll trouble is based mostly on when the main game asks you it import endgame save from varnhold, not the story continuity. Also you will be roughly at the levels that you go through varnhold with, so you will have general undeerstanding of how the game plays.

It is quite abit harder than the main game and being a low level adventure you have a limited amount of options to remedy that, so i would suggest a custom party with atleast 2 pet classess.
My personal favorite is ecclestiheurge with Erastil / Nature which gives the best healer in the game a free pet.