You can not only downgrade the cost of the resort, but even shorten the trip a day or so. You can buy the base ticket for x number of days and add the parkhopper later, even on site after you've already used the ticket. I believe its $45 to had the PH option, no matter the length of your ticket($135 total).
If you don't get the PH option be sure you go to the MK on a day when you can see Wishes and night time parade. On our trip they did not do both every night, so check the schedule (you can find it under the Theme Parks tab on this site).
We took my 5yo

nephew for his first visit and had 4day PHs. Since you plan to return in about 5yrs or so I'd say you can skip the Animal Kingdom. We enjoyed our time there and the Nemo and Lion King shows, but the Magic Kingdom is where she'll probably want to be and there is just so much to do there. We went to all 4 parks, but nephew asked to go to MK everyday and we took him.
Bottom line, if your trip is 4-5 days and you only go to Magic Kingdom, she will feel like a Princess and have very special memories with Mommy and Daddy.
I would be sure to take her to a couple of character meals, breakfasts are the least expensive. We took a 5yo Princess to 1900 Parkfare dinner at the Grand Floridian. She brought her dress from home, but we got let her pick out a couple of inexpensive accessories on site (tiara, necklace, bracelet?). I hear very good things about the Playhouse Disney breakfast and lunch meals at Hollywood and Vine (you'd need an MGM ticket for that one). Chef Mickey's is fun and also in a hotel so no park ticket needed (Minnie is there).
Best wishes and just remember, Disney, Characters, MK and she will light up before your eyes. Take your digital camera!