As you haven't already bought the bike, I will add to some of the comments above.
Outside of North America and Europe there's little need for more than 500cc and you can get away with less. And you will never find anyone on a rough trail wishing their bike was heavier.
I would always advise carrying an inexpensive emergency storm shelter in case you are caught out, but a proper tent with sleeping, cooking gear and food adds considerably to the weight. And some of the time during your 15 months you will be in the winter months when it's an awful long time dark with not much to do in a tent once you've made your meal.
Your challenge is to find a bike that's light but sturdy enough to take the weight of your luggage. If you get an older less expensive bike you will find the carnet costs cheaper, and you can put the money towards staying in hostels rather than camping.
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"For sheer delight there is nothing like altitude; it gives one the thrill of adventure
and enlarges the world in which you live," Irving Mather (1892-1966)
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