N.,
I would put the baby in a short sleeve top and pants. You can always add a blanket, and it is easier to take off than having to undress or remove clothing. Plus, if it is that hot, you can put the baby in a really cute short set and just add a blanket, or what the heck, just a onsie! Remember that if you are hot, then the baby is probably hot. And vice versa.
When my oldest was bor, we put a long sleeve sleeper on him(he was born in Feb), he got spit up all over it and all I had with us was some ugly outfit someone gave us in the hospital. Needless to say, it was not weather appropriate. But we made it home fine. Trust yourself, you will know what to do. Good Luck and hope this helps and sorry if it doesn't!
Congrats!