I think you are on track with your selections. I wouldn't worry to much about changing those factory speakers if you are adding a subwoofer. I used to have the exact setup in my car (same size and shape as the VW) and it sounded fine. The only people that will tell you to upgrade the main speakers are those that want perfect sound. For most of us this setup is really not bad and its a really easy change later if you feel the need. The 5 channel amplifier is a good choice for you. I don't know anything about the particular brand you have picked out but I would recommend one that has a remote level adjustment for the subwoofer channel. I have since upgraded for this reason. Just make sure the amplifier has high pass filters for the 4 main channels.
Alex Lear
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Alex Lear