I have only heard one HU capable of mp3 format, it is made by clarion, I believe it sounds great my friend purchased it online from etronics. it plays Wma's also
Clarion CD/MP3/WMA Receiver
it has 2 sets of RCA out puts, the rear set can be switched between Subwoofer output and full range rear output (default is subwoofer, at 120hz i believe) an Auxilary input and Dual zone. it's price is 159.99 plus shipping, there is other models that play mp3's that are cheaper, but this is the only one i have heard, and i believe it's well worth the 159 etronics has it listed has.
As for amplifier and subwoofer, there is a lot of choices, look for an L5 8 inch kicker square, they are BIG bang for the price etronics has it listed at 89.99
02S8L5
it is a dual voice coil subwoofer, if someone has the link to the talked about car that is in the crazy 8 class that hits close to 150 decibals, with 1 8 inch square please post it...althought it may be the L7 8 I am not sure.
However you may want to check other websites as they may be cheaper, etronics is the only site I personally have had experiance with. As for amps, and staying under the $500 range, I am not familiar with amps cheaper than $300...sorry but I can't reccomend something unless I hear it....peace
Edit. Also the 8 inch square you may want atleast 2 cubic feet for the box, and may sound loudest ported, sealed is for more accurate sound. ported really doesnt sound much different, just louder. And if you dont have a credit or debit card, find someone who does that will trust an internet transaction, its well worth the money saved.