I have been a professional car audio installer for 18 years, and I have to say that there are NO unimportant parts, I hate to use a cliche', but a chain is only as strong as it's weakest link.
The Mp3 decoding decks are still pretty new, first revisions, I have been a fan of Alpine's car audio units for a very long time. They will have one available in June, CDA7878, but it will be in the $500-$600 range, if you can wait a few weeks the newest 2001 modles will be out, Most car audio companies show their new units at CES and then release them in the spring, and from I can tell, most of the companies will be lowering prices. This means that you can get the newest units with more features for the same or less money. panasonic builds a very good CD player, they last forever and are very reasonable, and don't forget, your local specialty dealer will usually match or beat Crutchfield's price, I know we do usually.