I can recommend Hitman (full season) at € 10-20. It's actually a Hitman game done right, and the content you get at that price makes it a bargain. Replayability, or at least the freedom of approaches, is high. The mechanics are more common sense now as well. However, I have this silent assassin achievement OCD, which might not be the the most enjoyable first approach though.
I'm usually an all in-er. One task / one game at a time with 100% attention and e.g. a full day at time. However, I realized that strategy failed due to the sheer content of Witcher 3 (GoTY). There's just an endless amount of content, no normal mortal could force even in a dozen all day sessions. At a casual pace the game could probably last as a side project for a year. €25 is a bit much, but worth it.
Tropico 5 complete at less than €10 is also a bargain, even though 4 stays more true to the series.
On Steam I would recommend Crisis in the Kremlin at less than €5. Difficult game. Paradox titles don't seem to have a great sale this time around. Would say buy only if you intend to play now. But then again, they might make a price hike and do a maximum 50% off at a later date as well. Mandate of Heaven is 20% only, they'll likely go down to 50% even with a hike, in the future, which likely will be cheaper anyway.
Don't know what Outlast is, but seems to have good reviews, and won't say no to a chance to expand my linux game library