Don't buy a rug until you have measured the foal that very morning because she will have grown over night! A rather wimpy filly (half sister to below) I had last year started winter in a 56" blanket and ended up in a 64" one over four months.
Rugging depends on the individual as always, and weather, in the UK I would be more inclined to rug than I am here, just because of the incessant rain.
That was a 6mth old arab x QH foal, she had a coat that was 3" thick and never, ever, looked, or felt, cold or miserable through 4 months of snow and silly minus C temperatures. She was never rugged and came out of winter very well.