Bike in auction condition is least likely to start, 1. The battery would be dead or missing and 2. CDI will not be there.
Since you are from Islamabad, visit Basheer Ustad at Rawalpindi Tipu road for information regarding availability of parts. Although, someone told me he doesn't work on a GS500 any more so I'm not sure if he can be of help or not. Another workshop I heard of is near old Airport. But first things first, get a bike and then search for the parts you need.
GS500 is an amazing bike. I sold one recently after having owned it for 10 years