First of all, get a Guest Assistance Card. Even though he may be a sensory seeker in some ways, he may have issues with crowds, or strangers/people getting too close to him, or whatever, so having a GAC card could help him by allowing him access to a place/location while waiting for a ride or attraction where he won't be over stimulated.
He sounds fairly high functioning, a lot like my DS was at that age.
Echoing (ecolalia) is a good thing too. It means he WANTS to communicate, and it helps him LEARN how to communicate, he just doesn't know how yet, (other than to echo people). He should hopefully grow out of this in the next few years with proper therapy at his school.
I highly recommend renting a stroller there. I agree that the hard plastic seat/sides, and the overhead cover, is great for kids with autisim. He'll like it.....it's like his own little space.
I can't really recommend a specific place to stay. Everyone's needs are different. However, some of the quieter resorts are: Fort Wilderness, Saratoga Springs Resort, and Old Key West come to mind.
I'm not sure what the blanket is for. I've not heard of it.
If you need any more help or advice, I'll be here on the boards!
