Tell me straight, will I enjoy DS?
Actually, nobody can tell you that. It still polarizes even die hard fans.
Personally, I recommend to play it just for the experience (plot, atmosphere) and the stuff that is considered better than the older two games (physics engine, dynamic shadows and graphics).
The Cradle level is a must-play like many have expressed already and other levels are also very good. I surely wouldn't want to miss out of those.
On the other hand the levels are way smaller than in the first games and it is definitely dumbed down a bit for consoles (Xbox1 and PS2 capabilities) but one can live with than and there are some interface tweaks for PCs available. The only real problem I experienced is the "jump bug" where Garrett kinda slides around after doing certain jumps. They never fixed it, sadly.
And, yes, there are also some fan missions for it - playable with the GarrettLoader tool. Mind you, because T:DS received mixed attention from fans it never had the masses of missions like T2 so your time with it will always be limited ;-)
TTLG is usually at the cutting edge regarding mods so I can't really add to this.
However, if you are through with T:DS then check out "The Dark Mod" for Doom 3 - while it has no real Thief storyline you may come to the conclusion that it is a better platform for "thiefy missions" than T:DS...