Summer was born in October & didn't have to wear too much through the days, but nights were still pretty cold.
I'd plan on it being about as cold as it is now, since it'll be the other side of winter (& I dunno about you, but I'm bloody freezing on our sunny coast!!)
I'd get the 4 - 6 0000's, but more 000's. Bubs will be in the tripples longer. Summer didn't move up to 00's til she was between 6 & 7 months, so even a big baby would be in the 000's for 3 or 4 months I'm thinking
HTH