Where do you live? If the weather's anything like Sydney, I suggest that if bubs is going to be born in January you'll not need to buy any blankets. Or maybe just one in case you get a freak cold night. (Depending on where you are, of course...) For the cooler summer evenings a flannelette wrap does just fine over a muslin one. Then, once the weather starts to cool down you'll know what you need (i.e. you'll know if you're going to transition to a sleeping bag or if you are still wrapping then you'll know you want a couple of blankets then).
And 20-30 growsuits?!? Only if you're planning to wash once every 3 weeks, LOL! Phoebe isn't a chucky baby, so we got by on about 5. And my DD who was born in December didn't wear a growsuit til she was about 6 months old. Singlets are definitely the go for summer babies IMO (They're cheap too, so that's a big plus!)