AFAIK N-Gage is Series 60 OS 6.1 phone with some special extension libraries for games.
So basically if you make the app with SDK 1.2 it works in N-Gage & propably all other OS 6.1 phones, and also in OS 7.0 with some limitationa (some functionality is changed and might not work)
I also have heard that there is special SDK made only for N-Gage, haven't have any use my self for it, so never seen it really. Anyway I would suppose that if you use this SDK and the extension API's, you can't run the app in other phones anymore.
I have a question, since you mentioned the SDK 1.2. Can you tell some major differences/improvements SDK 1.2 has over SDK 1.0 ?
symbianyucca is right, there is an N-Gage SDK but you can't get it as a private person ( check it out on http://www.n-gage.com/en-R1/developers/ ). And as far as the N-Gage emulator is concerned, you should be fine with an S60 emulator that comes with any Series60 SDK, since N-Gage is Series60. Anyway I suggest that you get real hardware after some progress in development because the emulator is no good to develop a game until the endversion.
PS : Maybe we can keep in touch, I'm developing a game for Series60 myself right now. What kind of game do you plan to make?