For example, how many spells are there? Without a list, it's hard to know if buying a particular spell is useful or whether I would rather wait to get something else.
Okay, that's understandable. I had a similar problem with Baldur's Gate, because there was no list in the manual either, if I remember that correctly.
The spells within the game are only a fraction of the available P&P spells, of which some cannot be implemented in a game due to their "meta" characteristic.
On the other hand, the number of experience points increases within the course of the game so that you'll have a relatively good amount to spend later on.
The only problem are imho spells found only or almost only in areas you can't visit later - which are, due to the design decisions, quite a lot.
As a general rule, I rather "bought" spells which would support survival of the group/party, in any way, including weakening the enemies. With focus on offensive & defensive spells. In my opinion, these are point well spent.
About the "helpful paw": A (rather small) group of people in the officioal forums complained that the animal is lying as if being killed, rather than just disappear. This distracted these people (including me, which was the reason why I never used that spell).