2002 VW Jetta 1.8T 2.0L Turbo Alternater Problem

I took my 02 Jetta to the dealer and they told me that it needs an alternater and alternater belt. They quoted me a price of $698 :( for this.

My regular mechanic doesn't work on VW's and he told me that this was something that most likely needed to be done at the dealer because of the computer. Does this sound right?

I'm wondering if it is possible to have this done by a lower priced mechanic shop if I purchase the alternater and belts myself. I haven't been able to receive a straight answer and before I give a ton of labor cost to the dealership I want to make sure it isn't something that can be done by someone else. My mechanic told me that if I found out that the computer didn't need to control the alternator he would purchase it and put it in for under $400 but he couldn't guarantee it would work because of the computer.

Any information that you can give me would be greatly appreciated. Thanks a lot.