i think you may be able to get a converer to use the changer but i am unsure and cant imagine they would be too cheap.
JVC make good budget HU's. If you get an mp3 disc player HU and you wont need a changer as mp3 discs hold 6 discs on 1, thats what i use, its up to you though.
The standard speakers should be fine but upgrading to slightly uprated ones may be preferable depending on the rating of the standard spks, VW audio is generally pretty good though.
Are you thinking of adding a sub for extra bass or were you looking to stick to normal speakers?