Sounds good, I hope to get down to that part of the world myself in June.
I'm not the world's expert (Ive been to France/Belgium/Luxembourg once earlier this year), but the motorway from Calais down the Eastern edge of France was quiet when I went (late May) and an ideal introduction to remembering to look over your left shoulder.
Roundabouts feel VERY strange, and so do T junctions where you have to stop. Trying to get your head around where to look is the worst bit.
I fixed (zip ties) a cheapish car sun-visor mirror to my left hand mirror stem. They are sold as kiddie mirrors sothat you can watch kids in the back of the car and it isd supposed to clip on the sun visor. Only a few quid in Halfords, but it gives a wide angle view of that blind spot over your left shoulder. I found it really useful, especially since I tended to slow early for junctions so I could have a good think about where I was supposed to go, and where to look for traffic that might try to kill me.
I found that reciting a mantra (ride on the right) every time I pulled out, from a junction or from a petrol station, was useful. And then again (ride on the left) when I got back to UK.
Advice I was given was not to try to go too far too fast. Dealing with strange road signs, riding on the wrong side, etc all tend to slow you down, so blasting all the way down to Geneva may be possible but not very enjoyable. The advice from Gromit to take a few days and use the back roads is very sound.
As for what to take - depends what you you are doing when you get there. Pack light, but take what you would need if you were doing the same trip but in a mate's car. One thing I did was to get some BIG poly bags (bin bag size). Everything went in them inside my bag, with the top well rolled over. Even if my bag leaked I still had dry clothes.
I also had Gortex gear that was a godsend. Other people were faffing about trying to get waterproofs over leathers, and then get them off again. Gortex is the way to go.
Good luck, and enjoy.
Cousin Jack
(a member of an oppressed minority whose legitimate aspirations to nationhood have been brutally suppressed by the Anglo Saxon invaders. Remember An Gof !)