Hegen go for 150. If breastfeeding go for slowest teat, even if expressing milk they also don’t need to size up as fast as compared to formula as bm is quite thin. I would get 1 Hegen and 1 of another brand. See which one baby can accept then can get 3-4 bottles so you don’t have to keep washing after every feed. I won’t recommend to stock up on Hegen before trying as it’s very expensive. I got Hegen and Avent free gift, baby rejected Hegen.
I started my baby on 150ml and slow teats. no issue for me. suggest you start with maybe 2 bottles to see how your baby takes to it! if they have no issue with Hegen then can start to buy more
From what i read, best to buy the 150ml bottle so can use longer. Can buy different teats extra slow & slow. And buy 2 bottles first. Cs some baby fussy