My M2 came with a 4 speaker system (2 front, 2 rear), and the std OEM crappy MP3 player. You can add an aftermarket amplifier and upgrade the speakers as well, but the problem is that you'd need to get signal into the amp, as well as a remote turn on to switch the amp on and off with the factory deck.
You could use line out converters, which are generally cheap, but don't offer a signal sensing ability so it acts as a remote trigger (relay) to switch the amp on. There are one or 2 though, that do exactly this, I found a Peripheral device called a SVEN2 which gives you 2 channels of line out converter, and a remote trigger as well, and it's fairly cheap too. Only problem is that it doesn't provide any sort of tuning ability or EQ functionality.
If you need that sort of functionality, you could look at a JL Audio Cleansweep, or a Rockford Fosgate 3 Sixty, or Massive Flatline, or *drool* Audison Bit 1, which will give you quite a bit of tuning ability as well as all the OEM integration that you need to connect aftermarket amps etc.
Eventually, I got a dash kit, and trashed the OEM head unit, and have now gone with a Kenwood head unit, and have also added a pair of 10" subwoofers, changed the front speakers, and added an amplifier to drive it all.
Good luck with whichever route you decide to take.
