Deus Ex GOTY is pretty cheap now at 1.74€. I know a lot of people love it, but is it really as playable today? I remember trying it out some years ago, and I remember how sluggish I found it. The character seemed to move so slowly. Maybe you can upgrade your speed or something later in the game? I really want to like it.
Something like System Shock 2, Thief II, or Deus Ex are still considered some of the best games of all time, and are very playable today. I played System Shock 2 for the first time very recently and I was blown away not only by the shear quality of the game, but by how much I personally enjoyed it.
It just depends on you. These games aren't designed to hold your hand, but for you to use some rational thought and puzzle out the solutions to the problems the game puts in your lap. Prepare for a bit of a learning curve as your brain wakes up out of the Assassin's Creed and Skyrim-induced stupor.
All three of the games I listed also have awesome modding communities. All three have massive fidelity overhauls, along with plenty of other modding choices. Here is System Shock 2 after all the mods.