Most of the patterns are worked out with 8ply and 4mm needles though but check first before you buy the wool
That wouldn't vary due to the temperature but I imagine you wouldn't need full longies, rather shorties or a soaker.

You can knit in 12ply but then you need to adjust the pattern as it makes a much bigger size. You could also knit in a smaller ply but you'd need to cast on more stitches to make up for that or you'd end up with dolls clothes.

Not sure if that helped lol!