New player going BT questions

I just got the space marines half of the leviathan box and im thinking of going BT for my first army.

They just look the coolest imo.

I have a few questions though

1.- since it seems i cant use the librarian, is it worth to buy the black templar bits box from GW ? Would this help in converting the librarian to a chaplain and would anything else be usable for the rest of the leviathan box? ( rather ask before I build the models and paint them )

2.- I like the black & white paint scheme of the black templars , however , I have seen pictures of BT units with red helmets , or main armor yellow or red instead of black ( only the black cross decals helped me confirm they were BT units), this seems confusing , cam some explain how this works?

3.- you only have the leviathan core as a BT force with the librarian converted to a term chaplain , and own the BT bits/upgrade kit. What's the next purchase ?

Thanks in advance