i reckon a sleep bag is a good investment and I LOVE love love my merino kids one. i got the duvet weight one from ebay and second hand (in good nic) i paid nearly $90 for it. BUT, it is sooo worth it. it is from newborn to 2 years.

if you can invest in a good one, i recommend this one...the merino wool is largely self regulating for baby (ie never gets too hot).