Its an 84 stang, hell no it won't hurt, I hate to insult that particular car, i mean really hate, since its the first generation of the Saleen and I respect it a lot for that but it is definitely one of the less desirable models of mustang for the classic car collector the body style isn't as desirable neither is the engine that came stock really. By no means would an after market stereo hurt its value (Keep in mind this is assuming is not a saleen or a special edition of some kind), it might actually help since the factory stereos in those years were awful. The people who that car is most desirable to is racers since it weighs very little and parts are readily available, and to them the stereo won't really matter either way. Besides most of the time its easy to put an aftermarket stereo in and swap it back out later. However I think you might have to mod your dash a little.
No it wouldnt but if you want to keep it close to stock its up to you. I have two friends one with a 77 Caprice classic and his Cd palyer is in the glove box another friend has a 64 impala all stock with the radio mounted under the dash area