Hello Amber!
Congratulations on your upcoming visit! You are sure to have a magical time with your son's first visit and 5th birthday. What a great way to celebrate!
I definitely think that
Disney's Memory Maker is worth it, especially for such an important trip. You'll of course want to cherish all of the magical moments from his very first trip to Walt Disney World Resort and Memory Maker makes it so easy to do so. I love that it includes unlimited downloads of all of our captured moments, but it also makes sharing our memories easy and creating some great gifts for the family a breeze. As far as the stroller question, yes, I rented one for my son all the way up until he was about 7, even though he quit using them at home at around 2 or 3. It's estimated on average Guests walk 7-10 miles per day, so their little legs get tired quickly. We like to order from outside companies such as Kingdom Strollers because of the comfort, storage, and ability to use it outside of the parks. Regarding transportation, it's solely up to you. I like being chauffeured around, so we almost always use resort transportation, but there are plenty of times where a car would come in really handy such as early morning dining reservations, the need to get to a location in a set time, or to have the ability to grab some extra groceries.
Have an incredible trip, Amber. If I can offer you any more assistance, please let me know.
Magical Wishes,
Lauren