My daughter was 10 months when we went last year and still having milk 3 times a day, i took a tin for morning and night in the hotel room and a box of sachets to take out during the day. I used to take a small flask of hot water and a sachet out with me and just mix it when i needed it.
I agree that 60 cartons would be very heavy and you risk them leaking, we did buy some cartons after we got though security at Gatwick but just enough for on the plane and for coming back aswell to make the journey a bit easier.
My little girl likes warm milk so just had to put it in an ice bucket for a few minutes to cool a little, but if your little one normally drinks it cold i can see that boiling a kettle and waiting for it to get cold may take a while.
Try not to worry about it, i'm sure it will work out fine whatever you do, taking a baby was alot easier than i thought it would be.