The full list of content packs from the first two games is this:
- Hitman
- Hitman GOTY content
- Hitman 2
- Hitman 2 Expansion 1
- Hitman 2 Expansion 2
To get all of the content, you need to buy
Hitman GOTY Edition and
Hitman 2 Gold Edition - these will get you all of the content in the respective games.
If you are on console, you must install Hitman 1 and redeem the "Legacy Pack" through the game's menu, which is the DLC that will let you play the levels inside Hitman 2. You will likely have to do the same to Hitman 2 as well when Hitman 3 is released.
If you are on PC, you do not need to do anything other than own the games. If you're going from Steam to Epic for Hitman 3, you need to connect your Steam and Epic accounts on IO's website but there's no need to ever install Hitman 1.