References for joints?

That is going to depend a lot on what sort of doll you are going to make and what you need out of the joints.
For a start you could simply look at as many different existing BJD that you can get your hands on. If you get the chanse to play around with them, all the better. Is there any meets ar convetions in your future? Or any somewhat local doll owners that you could ask nicely to let them check out their dolls.
If you can't do that you can look at things like restringing tutorials to get a look at the parts.
Looking at the project journals here is also a good way to get an idea of how other people make them.

If in doubt, you could strat by making some basic ball joint pieces as a test to play around with to get a feel for it and also try out the sculpting materials you want to use and then take it from there.

 

Search thingiverse for "BJD", you should be able to download 3D models and see the joints that way.