i dont know much about CSM, but abbadon sounds good. and the s4 bolter cannot hurt the t8 nightbringer.
if you do tau (i know you said you wanted chaos, but still), you can get a Twin-Linked Railgun for 70pts (Broadsides), and you can use hammerheads as they might be a little more resilient and they both wound on a 2+ against the nightbringer (haha, take that t8!). But tau have got a lot of high- strength guns, mainly railguns, so personally i would use tau every time. oh, and they're ap1 is well in case your opponent gets some notion about armour svs (do C'tan get armour saves - i don't know?)