In steam, this DLC costs currently € 1.5, but I cannot see it on GOG, as it appears to only be in the deluxe edition, meaning I'd need to spend 9€

Is there another way to get it?

Well, its not worth it, actually. All this thing is do is
1) Give you some obvious hints about that you should do like "Go to Prince X and get the key"
2) Its collects rather rare Energy Spheres and charges itself up to 100%,as well as charges slowly as you gain EXP.
3) You can spend energy it collected to some tasks like "give me some extra food" or "gimme some gold" or "boos my stats a little". Price to get that stuff is high and effect is small.
4) Forgot to mention - its also can warn you about hell hounds going after you. This one feature is usefull, not critical, since these fights is unavoidable, but its gives you enough time heal and prepare. And, these hounds is not most scary demons anyway.
Overall, this thing is not changing anything in game. Its nice to have if you have it, but to pay for it...nope.

An extra skill point for your whole party can be pretty significant. This costs the full 100% amulet charge to do, so you can do it probably several times by the end or more. The downside is that the item permanently occupies your main character's amulet slot from the start of the game.

You could probably play and finish without the DLC, but if you wait for a big enough sale, it could be worth it to upgrade. I'm sure pirated copies exist, but I have nothing against this publisher and I don't wish ill on them. Considering that there are difficulty levels, playing without the amulet is probably a fraction of a difficulty level harder, if that much.

The Talisman is a net gain when it comes to skill points, so there is little reason not to use it. But it makes Gaulish even more of a sheep since he has to wear the artifact instead of a neck item with stats.

If memory serves, for most of the game, it is more effective to get gold (20% energy) and use it to buy skill points from the trainers. I tended to only turn energy to gold one at a time when I got close to 100%, since the amount of gold gained seemed dependent on level. (or perhaps playtime?)