Raymond,
You are correct in that Soundtraxx DSX decoders are sound only, and have pretty specific sounds on them. Because of the specific sounds, there are several different versions available, based on your particular locomotive needs. You will need the Digitrax (or other brand) motor decoder to go with it.
Soundtraxx also just came out with new DSD-101LC sound, light & motor decoders. While less expensive than the DSX decoders, they are intended to be low-cost, and may not have the level of sound quality you might be looking for. But they should be superior to their older-generation DSD-100LC decoders. Also, I don't think any of their LC decoders have silent motor drive. I have a -100LC in a Spectrum 4-8-2, and the motor has a bothersome growl to it. I suspect it's because of the lack of silent motor drive (all my other locos have NCE decoders with silent drive and none of them growl). I'd like to replace it with a Tsunami, which does have silent drive and better quality sound.
Also, did you consider a Digitrax sound decoder? I don't really know much about them, I just know they exist.