I used Avent bottles with DS and they were great. No leaking etc except for a couple of occasions where I had not seated the teat correctly (sleep deprivation is a wonderful thing. Once I forgot to put the teat in LOL!).

We used S26 Gold (after 4 months of research by DH, yes he is that type of person!). Some children do get very constipated with it. DS started to get constipated with the follow-on formula but that's another story.

Apparently the Nan formula is the closest in taste and texture for breastmilk but haven't tried it personally. Does that sound weird?

I would suggest that you buy one tin of formula of whatever type you choose just in case bubs doesn't take to it. Try not to swap and change formulas too much so it may upset bubs tummy.