That's the trouble with the internet; you ask 10 people a question and you'll get 20 different answers. The reality is that most of the gnarly old stagers on here would do a trip such as yours on whatever they've got rather than starting from scratch. I've done trips this year on half a dozen different bikes ranging from a mid 70's 125 Suzuki two stroke to a Honda GoldWing - including a KLR650 which I quite like as a travel bike. It's fairly basic but it kind of "fits" and I feel comfortable riding it long distance.

A KLR (if you could get one), a DR650 (again if you could get one) or a XT-Z would probably be the mainstream choice, but that mainstream would consist of people who have years of experience behind them. For someone starting out I think weight is the biggest hurdle you'll face. By the time you load any of those bikes up (and with the extra power of the 600s it's easy to keep piling stuff on) you'll be struggling at low speed. Maybe if you were built like a night club bouncer it wouldn't be so bad but if you're not it can become a real struggle at times. It's easier with experience but you have to get there first.

That's why I suggested a 250. Sure, you can't mix it with the Porsches on the autobahn and it can be a bit of a grind on long mountain climbs but the lack of weight will make riding it a pleasure the rest of the time. Use soft luggage (there's quite a few decent makes around) and if you can keep it down to around 15kg (I'd have my doubts ) you'll feel the benefit when the tarmac runs out (or breaks up). Personally I'd avoid loading the bike up with aftermarket parts - soft luggage (depending on the bike you might need some kind of add-on to keep it off the exhaust) and thinking about fuel range would be it.