When they're that little, sometimes it's easier to change their diapers when they're wearing one of those outfits with an elastic bottom. It doesn't have feet, but it's long and the elastic opening at the bottom keeps their feet covered - plus the sleeves have the fold over option so they don't scratch their faces. Since it will probably still be pretty warm outside, you might want think about using one of those or even the t-shirt with long pants set (Carter's has lots of those). However, remember that you won't be outside with your newborn for very long - just to get in and out of the car - so you'll probably want to think more about the temperature inside the car, and then inside your home once you get there.
I brought my daughter home in early May, so we hadn't experienced a true heat wave yet, but it was still warm outside. She wore footie pajamas and a hat because the car and the house was cool. After a few days, when we started walking down the street and back, I put her in a onesie and made sure she was completely shaded while we walked (no sunscreen until six months old). The onesies didn't bother her umbilical stump because they were pretty loose - in fact, I would recommend those over pants since the elastic might rub (if you're worried about it that is). Until the stump falls off, you'll probably have to fold down the front part of the diaper so it doesn't rub on the umbilical stump, and the onesies are loose enough to make it comfortable for the baby.
Whatever you dress your baby in to go home from the hospital, I would recommend that you buy at least ten onesies for daily wear. My daughter wore them every day, and the elastic bottomed nightdresses at night, until she was several months old. Good luck with your new one!