Hello, Welcome,
1. BMW makes a luggage rack and topbox that can have matching panels and a matched key. It is made by Shad and you can but a non-BMW version from Shad at a lower price but without the matched key. There is also a BMW tunnel bag that gets mixed reviews. Givi did make a sidecase/pannier rack for the BMW but is seems to be discontinued although some can be found at various places. Many soft panniers and bags will work on depending on what you are looking for.
2. As far as I know you need to have BMW reset the light or buy a GS-911 tool that will work with the C650GT/C600 as well as other BMW motorcycles.
http://www.gs911usa.com/
Between this board, the BMW Scooter group on Google+ and the info pages on WebBikeWorld you should be able to find additional info on many questions and more on these...