Hi, short and simple question: can anyone point me to a good caster build in the game?
Preferrably who can take on the game at Blood and broken bones difficulty, and preferrably with a bit of detail when it comes to skills and progression.

Hi, short and simple answer: no

You see, it is about each one player how it like to play, I like to have a sign build, and very little I use potiions/oils, this make me feel very easy to play the game...so you must test and keep what you like.

Pick your own and make it your own. But here's some examples to get you going:
https://witcherbuilds.wordpress.com/category/sign-builds/

Thanks for your comments, I will have a look at the builds. :)

The linked Pyromaniac build is very good, I finished the game with a very similar build.