I don't follow. How did you go from having played Thief 1&2 and wanting to play Fallout 3 to wanting to buy the Orange Box? Those titles have just about nothing in common with each other beyond the first person perspective.
1: There's almost nothing to learn. Point on enemy, click on enemy. OK, it's a little more involved in that but Half-Life 2 and TF2 are just plain shooters. Portal is a little more brainy, but the game starts out simple and introduces new obstacles one by one.
2: Yes.
3: Have you played Thief - Deadly Shadows? If not, you should.