If it's a Legacy wagon and not an Outback, it's a BK. BG is the raised roof version - it has a little bit of a bump on the roof compared to the Legacy. In the US, the only BG was the 96-99 Outback.
You can always check your VIN to make sure. It should read something like "4S3BK4...."
The "BK" part is your chassis code.
As far as the tophats go, I believe you can use WRX ones, but someone who has actually done a GD suspension swap may want to chime in on that.