Well, generally you cannot pre purchase a visa more than 3 months out from when it will be used.
You need the carnet to be current so as close to set off date as possible.
Your visa will need to be bought as you go, if you have two passports(good idea to get concurrent ones) then it is easir as it won't slow your travel
Turkey is buy visa at the border, you won't need a carnet, same with SEA. Thailand prefers a carnet, but not really needed
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