In that case you could wait for the next sale and grab the whole Sierra repertoire. They had them all (QFG, PQ, SQ, KQ, LSL) for peanuts during some sale last year, I'm sure they'll do the same again soon.
The QFGs are also generally more forgiving than the other Sierra adventures, it's easy to die of course but they tend to stay away from the nasty dead ends that made the other series so infamous, especially in the 80s.
The Dig fits almost all you requirements btw. It's not as straight SF as tuluse says - it's very subtle, but the game's overarching theme is mortality and how we cope with it, though almost all of it is in the subtext and very little is spelled out. It does have a fantastic feeling of exploration that you usually don't get done so well in adventure games. Graphics are technically aged but the direction is superb, as is the music. Ending sucks donkey balls, but until the last couple of minutes the game is superb. Very highly underrated little gem. It's not a very easy game though, and some puzzles will drive you nuts if you're new to the genre.
Steel Sky and GR are both pretty easy games, and are good introductions I think. GR has maddening popamole action (LITERALLY popamole - fuck whoever thought of this) and I found it way too easy, but that's actually a plus for a beginner's game. No real exploration though. Grey Matter is also pretty easy, colours vary (cutscenes are awfully cartoony, actual game screens look superb), but good luck finding it anywhere legally.
Have fun with SS and GR and post your thoughts! AND DON'T LOOK AT SPOILERS. If you get stuck, ask for gentle nudges, looking up outright puzzle solutions is the best way to completely ruin any adventure game.
I meant Lucas Arts adv games from steam on a sale. QFG I already got from GOG.
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