@Urmas I have some questions for you!
1. You can use strong flour, but I'd rather use all-purpose flour because the result is "lighter" — both in colour and consistency.
2. Custard powder would be OK, too. As for the amount... the one I used — read: the one I happened to have in my pantry — comes in 63-gram bags. If used for its "original purpose", i.e. vanilla sauce (to be ladled over slices of cakes), it should be mixed with 0.4 liters of milk. Does that help you? Anyway, the filling isn't exact science. Aim for super sweet, super vanilla-y, gooey... thing. Oh, and here comes Edd China's top tip: the filling will solidify when it cools, so don't let it cool down to room temperature before you apply it on the dough. Keep it warm-ish, not hot.
3. Cardamom is for the dough. Two teaspoons, coarsely crushed. Pulla dough is such a staple here that stores sell crushed cardamom in small tubes containing the amount needed for a "family-size" pulla dough (i.e. 0.5 liters of milk, 2 eggs, yada yada).
Here is a HOWTO-video for (braided) pulla. Mind you, the dough does not need to rise quite that much — double the original size "and then some" is enough. But you'll get the idea of the consistency of the dough.
(Edit: for some reason, the board software won't allow for starting the video at x min y sec. SO: Pulla dough section starts @4min 28 sec.)