We flew with our 18 month old to Hawaii which was about a 4-5 hour flight. We did not purchase another seat,.. but we tried to plan for us to be flying while he was sleeping.. My suggestion is,.. get another seat if you can afford it.. but if you cant.. Get seats in the way way back near the bathrooms or all the way in the very front... We kept our son near the window because we did not want him to think he could get down and walk around.. Bring new toys,.. flip books and several crayons so that if they are dropped you don't have to worry about finding them! Red eye is probably the way to go,.. I think you'll be pleasantly surprised about how wonderful she will actually do! Bring something for her to chew on during take off to help her adjust her ears.. Lots of snacks! STICKERS!! Stickers were a life saver,.. by the time we got off the plane we were covered in them.. Also some planes pass out personal videos,.. Renting one would be a good idea,.. Now a days they have all kinds of movies and shows on them.
Last thought,.. Don't sweat it too much if she throws a tantrum just relax, shes a kid. And don't worry so much if someone gets upset over it,.. they probably don't have any kids, they'll get over it! If she only cries once I would still consider it a great flight! After all, can you really blame her? I hate not being able to move for hours at a time too!
Towards the last half an hour on our flight we were getting ready to desend and my husband put our sons shoes on and got ready to go,.. worst thing to do,.. my son thought it was time to get up and go right when his shoes went back on,.. If your daughter is anything like this,.. don't get her ready until you really are leaving!! This caused a lot of frustration for our son,.. luckily the guy sitting next to us was about to be a father and he was very patient and understanding. It was quite a good experience for him! LOL