Am sure there is a wealth of advice here on the site, so keep reading!
I'd just say as it is your first trip, it's no doubt imperative to you that the bike is reliable and easy to manage. If you are fairly tall, then the Transalp comes to mind.
If you aren't so tall, you can consider the Suzuki GS500. This is an air-cooled bike ( less to go wrong ) and has a very frugal, easy to work on engine.
They can be bought very cheaply. The Transalp is ridicously overpriced new, a better buy used.
Hope it works out.