Your advised not to use them below 3 months of age, due to risks of SIDS, I used them from when my son was 3 months old, they are so worth it. I never used them with my first, but i wish I had would of been easier in the long run. As for using blankets go for it, and most places you can't get them below 3 months of age anyway.
They also if you look around do have different togs for weather changes. I have 1.5tog and a 2.5 tog. My son is only a year but he's in the 18-24 month growbags, he can stand up and crawl in them. They are brilliant hun, but If i were you, I'd only use them from when baby can be in them, and there head can't easily be brought through the hole for the nec. plus there are also ones that have clips for under the arms to adjust the tightnest to reduce risks of SIDS.
hope this helps and best I found for price are asda and matalan x x x