I would guess that you are going to build a Grey Knights-army? (If you go with Space Marines that is.) From the little I have seen they seem extraordinarily powerful, although I do not know yet what weaknesses they might have, except maybe being expensive in points or similar.

Personally what drives me towards Warhammer the most, is it's very rich and dark back story. It is wonderful...

"That surface that tells the Heavens it hath an end, cannot persuade me I have any."