I guess the main question is like Alien asked, is this a solo project with the Triton being the main source for music composition? If so....
The Triton can do everything you want to do at this point on it's own. It has it's own sequencer, which I use to compose songs. It also has a sampler, fully automated mixer, and even high-definition recording tracks for vocals and other instruments.
If you are a beginner, you should focus on learning how to use to triton to it's full potential before looking for other external products. This could take you a full year, and that's if you work at it and study it diligently.
Things to consider would be good monitoring headphones, and a studio monitor system. Also, a good microphone for vocals. You can spend a lot of money on the computer setup and a software package like Cakewalk or Nuendo, but truthfully, the triton has a nice sequencer of it's own. If your budget is limited, priority would be on a studio monitoring system, headphones, & microphone.