If you're up for it, I'd suggest considering DIY. At least read the guides and see if it's something you'd consider.
If not, give Frankie at 20Hz a call:
He did the audio in my 2007 323i base stereo (HU, amps, sub). Tell him Ali referred you to him, maybe remind him of my car and mention you have the same kind.
The equipment will also run you some dough, I'd suggest considering what your main objectives are:
- Sound quality
- more volume
- more bass (how much more?)
- new HU functionality
Feel free to PM me if you want to chat in more detail.