EDIT: Regarding Hitman...
Exactly. It sounds very complicated.
The GOTY content for 1 costs me €18.38.
The legacy pack upgrade for 2 costs me €9.99 assuming I don't have to buy the "Full pack".
The Gold edition of 2 costs a whopping €89.99!
The standard edition €59.99. What's missing between standard and gold?
There's a silver edition mentioned that doesn't seem to be purchaseavle?
And there's a free starter pack but also a Santa Fortuna edition?
Eeeehhhh... Well thanks for attempting to clarify it for me...
THE GOTY content for 1 is frequently on sale, even outside of Steam, so it may be cheaper to get the content for 1 (of the actual GOTY Edition of the game), than to buy that Legacy Pack upgrade, depending on the occasion.
The Gold Edition is indeed expensive. It was also on sale recently, so you may want to buy it on a future sale. You can always get Hitman 2, and later on, purchase the Expansion Pass. It grants you Expansion Packs 1 and 2 (you would only be missing some exclusive in-game items, that you can currently only get with the Gold Edition).
The Silver Edition is no longer for sale, and it was a version that had the game + Expansion Pack 1. There's an upgrade option from Silver to Gold, which is essentially an option to buy Expansion Pack 2, for those who bought that version.
As for the rest, the Steam page page explains what they are.
The Santa Fortuna Edition is a not a different edition of the game. It's a pack that includes only the first 2 chapters of the game. The Full game includes these chapters, and the rest (if you buy the Santa Fortuna Edition, you can always upgrade later).
As for the free starter pack, this allows you to play the intro mission of HITMAN 2 for free.
And, since you own the first HITMAN, if you download the starter pack, it will allow you to play the HITMAN content on HITMAN 2, and the intro mission of the later.
This may seem a bit complicated, but understand this: essentially, all locations are DLCs. The full game has all locations (except those that are part of the Expansion Passes; only one is these is out). The Santa Fortuna Edition has some, but not all locations (but you can always buy the rest later).
If I may make a suggestion: download the free HITMAN 2 starter pack, and start the game.
If you do so, it will add HITMAN 2 to your library (it's no different from the full game, except you're missing some of the locations), and you will then be able to play the first location/intro mission from HITMAN 2, and you can play all the HITMAN content (except the GOTY content, which you don't own), with improvements. If you want, you can even purchase that Legacy Upgrade DLC, and get the GOTY content of the first game as well.
After playing through the HITMAN locations, you can always purchase HITMAN 2, in it's normal or gold editions, and play the rest of the content.
The content you already own, without paying any further cent, should last you quite some time, if you replay missions, play side-content, unlock challenges, ...