If I was you, based on having three GS's, I would get a GS1250 rather than a GSA - I don't understand why anyone wouldn't but each to their own of course.
Nav - get a Garmin XT
Panniers - BMW adventure panniers are worth it and I think they are pretty much the best. I have not found any that are much better.
Heated seats - absolutely. Freezing cold and heavy rain through Co Sligo last autumn and I was very pleased I had them! I ride most days through winter too and it is on more than it is not. Very useful.
Other bits - SW Motech engine bars, SW Motech tank bag, gear lever extender, adventure screen, Metzeler Roadtec 01 tyres, Keis heated top, toolbox is good too.
Nav - get a Garmin XT
Panniers - BMW adventure panniers are worth it and I think they are pretty much the best. I have not found any that are much better.
Heated seats - absolutely. Freezing cold and heavy rain through Co Sligo last autumn and I was very pleased I had them! I ride most days through winter too and it is on more than it is not. Very useful.
Other bits - SW Motech engine bars, SW Motech tank bag, gear lever extender, adventure screen, Metzeler Roadtec 01 tyres, Keis heated top, toolbox is good too.