My DS is nearly 2 and still in the cot due to him moving around at night.
he has been sleeping in a sleep suit since last winter (12 months) and they are fantastic!! He has lots of room to kick his legs around and move all over the cot.
Last winter we had him in a snugtime sleep suit which is a 2.0 tog (Approx$40-$50). Under he wore a long sleeve body suit under it and the heater was on.
ATM he is in a target Sleep suit and has a singlet body suit under.