I would recommend you stay with an 8 or 9 or 10 speed cassette hub. The six and seven speeds have freewheels most likely.

The cassette hub is way stronger and the rear axle in a cassette system is way stronger.

The Mu frame is about 1.6 pounds lighter than the speed frame but if you are going to add another 25 pounds in gear that probably won't make a difference.

All the weight on the back in panniers will make the bike fairly wiggly.

I would get a set of Tern minimalist front pannier racks and front panniers with a small top bag on the rear rack.

Have a great trip.