It sounds like you are trying to create something similar to what the .hack games had.

Back in the day I bought that game. And when I first heard the drums and the bass fade in with the lonely harmony and melody without an annoying cut, it was amazing! I myself always wanted to hear more games take advantage of this style of music immersion. Instead of the flat sound track the FF series made so common place.

This style of music to be placed into a game is rather hard to do. If you already have a music engine that can do this then all you need is a composer. Honestly you will be able to get exactly the sound you want, and have it custom made. I did some work in the past similar to this, but you will not find what you seem to be looking for in small bits. Such as purchasing a single bass line for a dollar... then a drum loop for a dollar. No. Usually in the world of producers they sell packs. Which means you will have like 70-1,000 "samples." These samples all vary in a range of time signatures. The audio sample lengths are all different. But this is not what you want to do, especially if you have never created a musical work before in a DAW.

You need a composer, or need to learn how to compose music yourself in a DAW. trust me it will make the implementation of the music a lot easier to work with.