I love Defenders and have always wanted one, but the right one for the right price has never crossed my path. I thought they all came with V8s, maybe it's just the USDM ones.
Anyway, it's possible to have a sound system with just a pocket mp3 player, but you will need to add least a small amplifier. No player will have enough power to play directly to the car's speakers. They are only designed to power small, highly efficent headphones. Some are not even powerful enough to power larger headphones. Otherwise install it just like any other aftermarket stereo installation, player (instead of stereo) to amplifier to speakers.