Hello and welcome

To reset the airbag light you need to either, get the car linked up to VAGCOM computer or buy a code reader from ebay. It might be the airbag its self that is faulty and they ain't cheap!!!!  The rust on the sills is quite common BUT, how bad is it? And where exactly is the rust? Is it on the sill tops where you open the doors or lower down on the outer sills? The strip that runs along the side of the skirts is a well known item to rust, is it this your talking about?
The VR6 has a cam chain, not a belt and the chain tensioners generally need changing at 100,000 miles and it is an expensive and very involved job!!!!! If possible get someone to rev the car slowly up to around 3000 rpm whilst you listen under the bonnet for the rattle. If it's well worn you'll hear it on tick over.
The dealers can charge the earth for a service. The best bet would be to find a good recommended independent garage.
Also, have a good read in the "VR6 Buying info" at the top of the page

Stu