U'll need an output display of some sort, a computer, some way of putting the audio through your head unit and some way of controlling it.
You can spend as little (well, not nothing) or as much as you like. You can put a laptop in quite cheaply (assuming u already have the laptop). If you want a permanent install, invertors will save you money, DC-DC power supplies are fairly pricey.
If you are on a tight budget you can go without a TFT LCD and get a smaller one with winamp plugin.
You could use a keypad or full keyboard/mouse to control it, its all up to you.
Your question is a bit broad, have a read around.