We took DD at 18 months. She was a new walker of a about 3 months and loved to walk at the parks. The best advise I can give is to take your time and go with the flow. You have no idea how she will react to everything until you are there. You will have a great trip if you don't set high, unrealistic expectations. Traveling with a little one is different then with just adults, but still wonderful.
I would highly recommend taking your own stroller. It should be familiar to your child and they will be more comfortable taking naps there. I also had great success with packing her outfits in gallon sized ziplock bags. I would throw two in the diaper bag each morning. They especially came in handy after meals or when she found a splash pad to play in. We went through three outfits one day, and ended the day purchasing a shirt and just having a diapered bum. Like I said earlier, go with the flow.
To get to ride the high powered rides is possible, but not as easy as before. We were lucky that DD would put herself down for naps in her stroller, so we would take turns doing the single rider/child swap lines at that time.
Also expect overstimulation on your child's part. WDW can over stimulate an adult so think what it does to these little ones. I have seen the most mild manner of children throw a full blown tantrum in the parks, and would put it down to being out of their comfort zone and being over stimulated. That is the number one reason I recommend your own stroller. It is something familiar to your child. W also took DD at 3 1/2 to
Disneyland, and used her stroller. When she got overwhelmed, she would ask to have the seat back laid down, and would pull the cover down to close out the park. My child who had stopped napping took a short nap almost daily there. The parks exhausted her mentally.
We also put a food order in to Garden Grocers to have there when we arrived. They deliver right to the front desk. We live in OR and went for two weeks. So when e arrived, I had diapers, baby wipes, swim diapers, milk and snacks for DD as well as a case of water. It was great only packing what I needed for two days in my luggage and having it all there. And I was able to have food she was familiar with.
Also, when I was planning, I found the mom's group on the WDW site (sorry, can't remember the exact name) was a great resource. It is a place you can actually send in questions and get expert advise from Disney Vets who are parents (some are Dads) on specific things. Must reading through older posts gave me great ideas. I even asked a few questions and got answers back quickly.
Have fun planning your trip. I am sure you will have a great time.