I would check with your airline. All airlines allow you to check your carseat for free with your luggage. Most will not let you use one on the plane. Even a booster, but a booster will fit under the seat in front of you. I took a booster on the plane in May, and the stewardess stopped me to let me know that I couldn't use it in the seat. It had to either go under the seat, which it did fit,or in the overhead which takes up room for other luggage. When we rented our car we included a toddler carseat in the price. The rental place had a ton of convertible, mid-grade carseats sitting there. Then we just dropped it off when we returned the car.
I just travelled with my 3y in June. She had her own seat on the plane and did fine. I had her sit in the middle or window seat, not the aisle.
For walking through the airport, I had a buddy harness. It has a long tail that you can hold onto like a leash. They sell them at Target,Walmart, Babies R Us. It was a life savior. You do have to take it off and put in on the belt at security, but it gave her some freedom and me some control.
My other tip is to check all luggage except one backback. In it you can have all the dry food snacks,blankets, etc. that you need. No liquids until after you are through security. Make sure to include at least one change of clothes.
Last tip is to have your child wear easy slip on/off shoes. Again its part of getting through security.
Also, since your child is under 5, you will be allowed to board the plane in the second group.