What would be best to iron out first?

1. Get a very simple UI up just to play mp3s, build from there.
2. Design a complex UI and build from the top down.
3. Build a good back-end to support what we hope to put in at later dates ... UI to follow.
4. Try and reverse engineer the current software, get the pants sued off of us and spend the rest of our days penniless or in prison.

I've typically done something like either 2 or 3 in other projects ...

Greg