[A4] Car Audio Recommendations

[tedzanos at freemail.gr]

Hello everyone,

The time has come to change my car audio system and also the lower driver side door speaker (since it's gone bad some weeks ago). I guess I'll need to change both speakers (which are by the way the subwoofers right?) or not?
The new system has to support mp3 cds and the my bad economic status limit the price to 150$ + 100$ for the speakers. Any recommendations?
Is the procedure relatively easy or should I just go to a technician?
The car is a '96 A4 2.8.

Thanks,
Theodore.