does the stock cd player play mp3 cds?

 

The earlier ones didn't, but around 2005, MINI upgraded the head unit to play MP3 discs.

 

i see. is it possible to buy a upgraded cd53 r50 from ebay and put in on the older models?

found some players for about $150.

 

Yes - all of the head units over the years are pin-compatible with one another, so you can swap a newer deck into an older car. If you go to European Auto Source (they're in the vendor directory), they have more information about what software version in the head unit is required to play back MP3 discs, but if you can find a stereo from any 2006 MINI, or any 2007/2008 convertible, they'll work in your car and will play MP3s.