I was thinking of buying Thief to play the fan missions.
but, it's been a while since I last played the game and I just want to make sure...

Thief II will only play Thief II FM's and is not backwards compatible with FM's for
the first game?

also, was the level editor included with the first game or did that come along with Thief II?
(and is it in the gog release?)

Yes, to play Thief 2 fan missions, you will need a copy of Thief 2, you cannot play them via Thief 1 or Gold. Thief Gold will only play Thief 1 or Thief Gold fan missions. Fortunately, Thief 2 will shortly be available here in GOG.

To play the fan missions, you do not need the level editor Dromed (which incidentally is included in this GOG release), you can do it via DarkLoader or GarrettLoader, either of which will do the job.

Thanks!