..does that keep me from completing the mission or only Expert achievement? Speaking of which, when you choose hard or expert is it required to meet all requirements to finish the level or only required if you want that specific achievement?

I'm currently at the end of the mines in the second mission and I lured a zombie to kill a guard that was blocking the way but I wasn't sure so I wanted to ask. I don't particurarly like the idea that I have to take the chance, finish the level then see if I "failed" completely or only the difficulty badge.

Expert is the way to play. More story, more places to explore, etc.

WWGD?

Choosing Hard or Expert will add new objectives to the missions, so there will be new requirements to successfully complete missions. They are not optional; it's not like an achievement, it's a difficulty setting you pick before starting the mission. Sometimes these new objectives are things like "don't kill anyone", other times they are entirely new objectives, giving you new things to steal, etc. It makes the game a lot more interesting, requiring more exploration and risk-taking. I went through on normal my first time as I was learning the game and then played again on Expert.

NOTE: I was playing the original Dark Project version, but I assume Thief Gold works the same way.