If I wanted to run say a Callidus assassin alongside a deathwatch group what books do I need - I know Dark Heresy has the rules for assassin acoyltes but what level of assassin would I need to make? Do I need the expansion (ascention I think?)

Failing that would a sister of battle be possible (she could bring different skills to the group like religion and inspiration - a bit like a chaplain) What book are the rules for them in?

If you want to play DW level you neer DH and ascension.

There are no rules however for callidius as PC. Only for a vindicare and imo he is broken even with spacemaries around.*

Rules for sisters of battle are in blood of the martyr.
The first have been in the inquisitors handbook but the most recent (and those that fit best to spacemarines) are in bom. There is even an item in there that gives the sister unnatural toughness, so she can take nearly as much as a battle brother. (nearly because she's got a weaker power armour.

*The reason I think he's better than a SM is that in the long run he gets more unnatural stats, he can dodge a nearly unlimited number of attacks, may dodge things other guys may not dodge, is sneakier and can do at least similar amounts of damage (to single targets) with a higher chance to hit (because opponents get a dodge penalty). On top, as every DH char, they got more skills and more talents than a SM.

Many thanks - so DH + Ascention (high level stuff) + BoM if I want SoB (at marine like levels)?

I realise that SoB won't be as tough/uber/whatever as a deathwatch but I think some diversity is called for. Essentially I was thinking of allowing our resident female gamer to opt to play the NPC callidus assassin that is in the free starter adventures (which is what I'll use for thegroup who may be playing) She has no ties (as inquisor is dead) and as an assassin is'ascended/superhuman/loyal to the imperium.

Last question : rules for neural shredder and polymorphine ? Or is it something I have to make up (which I have absolutely no reservation in doing - I just like to use the 'official' stuff if possible )