I prefer the little short rompers to onsesies, so that's probably what I'd put DD in and take a blanket (it's basically the same as a onesie though). I'd take a blanket for in/out because taking pants off and on if she was getting hot would be a pain. DD is usually pretty hot though.
Our pediatrician basically told us - once baby regains his birth weight and is gaining weight successfully, you can now start dressing him as you would dress yourself. If you are wearing one thin layer of clothes in 80 degree weather, don't put more than 1 layer on him, etc. We've been doing this for the last few days and it's helped tremendously with his overheating and heat rash. Good luck!
I'm in 90-100 degree temps- and I've been keeping LO in onesies plus I have 1-2 Aiden & Anais swaddle blankets around at all times- they're nice and light yet warm when need be. I double up with a 2nd when AC is blowing of we are out somewhere or I can use it to block any sun.
Re: 80 degrees, dress newborn baby how?
This is what we do-
With blanket: onesie alone
Without blanket: short sleeves, pants, and socks
This is what I do, too.