It is very expensive to go, but so so so worth it!
Are you planning to just do Walt Disney World? Or universal studios and Busch gardens etc?
From multiple personal experiences, definitely stay in a Disney hotel, the cheaper ones are great for kids as the themeing is incredible! So one of the All Star Resorts (sports, music or movies) or the pop century resort are amazing! And you get extra magic hours to the parks if you stay in their hotel.
If you're only planning to visit Disney & stay on property, the free busses are fab and you won't need to hire a car unless you really wanted too.
Disney it's self has 4 theme parks, Magic Kingdom, Animal Kingdom, Hollywood Studios and Epcot, and 2 water parks, Blizzard beach and Typhoon Lagoon. All are an absolute must in my opinion.
The way my parents always planned our trip was to do a water park on the first full day as the theme parks are long, tiring days and after a day of traveling it really does tire you out before your holiday has really even started. Then we did 3 days in theme parks, another day in water parks, 3 theme, 1 water, 3 theme etc. Just means you can have a break and slow the pace a little.
Outside of the parks, I'd recommend going to Disney Springs (the downtown shopping area) as well as the crazy golf courses, winter summer land golf and Fantasia golf. If I remember correctly they both have 2 courses. We used to do these on an evening after a day in the water parks as they close earlier than the theme parks.
We usually went for 2 weeks, but if you're wanting to do other stuff outside of Disney, I'd recommend 3 weeks, if it's within your budget.
I can't speak about spending money as I have not been as the adult yet, my parents always dealt with the money.
I would highly recommend booking the meal plan, 2 meals, 1 refillable drinks cup, and I believe 1 snack, per person per night of your stay. We would use the meals for breakfast in the hotel and our lunch and then have a more expensive evening meal, where we could sit and reflect on the day as a family and rest a little before the last bit of our day and the evening show.
I have sooooo much more I'd like to type but already feel this is a long message, but if you would like to know more or have any questions please feel free to PM me on here and I'd be thrilled to help you plan a magical holiday for your family!