I know several people that have gotten engaged at Disney, and seen several people get engaged in the parks... and each have done it on their own...
I personally would not spend 5,000 dollars on something that you could do yourself... take into consideration what would be the most special for just the 2 of you... what would be the most personal and lasting memory...
You could do something simple like take a evening Christmas eve stroll around the boardwalk or whatever resort your your staying at... and pop the question then... so you both have some time just the two of you to surround yourself in your moment... Simple yet really romantic...
I know several people that have gotten engaged in MK at Cinderella's wishing well.
One of our friends that got engaged in the Haunted Mansion on the ride. Why you wonder... that was the first ride that they rode together on their first trip together to Disney. So it had special memories.. He said that he knew she was the one, when they both said "Step into the dead center of the room" at the same time... and the rest was history...
We have seen several couples get engaged on main street during the fireworks,
Another couple we know got engaged, in Epcot under the trellis next to the Tea shop in England.
Another friend of ours rented a Boat for the fireworks cruise at the Grand Floridian and then after their families met them on the dock, and went to Narcoossee's for dinner afterwards.
DH and I were walking along between POFQ to POR back to our room early one morning, it was right along the river, and happened upon a couple getting engaged, it was so sweet... We asked them if they wanted us to take a few pictures, which we did...
Cali Grill is another spot we have seen a couple people get engaged...
Wishing you both a life filled with magic and pixie dust...