Well I have a May baby so not quite in Summer, but the bassinet is actually quite roomy, especially for a nb, so as long as you dress the baby appropriately I don't see the airflow being any more of an issue than using any other form of transportation for bub.

I bought the second seat straight away, but I have two so needed it anyway. Baby brain is getting to me, but I know that there is a particular adapter part that comes with the second seat, (I think it has something to do with the bassinet...) so if you want that option then you need the second seat straight away. I think if you google it you can find other forums talking about it. Sorry, my poor brain isn't quite functioning as it should be!