First, let me make sure I understand you correctly. You are already engauged. So you won't be getting engauged at Disney. If you are the easiest thing is to get your place in mind, what you want to wear and all that, then just snag a disney photographer. They are pretty good, besides they'll use your camera if you ask. You'd be surprised if you scope out random peolple carrying good equipment what a good pic you can get. Our Christmas card pic was taken by a stranger. I got the camera right with a few test pics then just snagged somebody carrying a SLR (canon of course). Now our anouncement pic (both the newspaper and wedding invite) was taken by boBQuincy you see on here. An excellent job I might add.
Now, if you plan on getting engaged down there, thats a different story. Call disney up and one of there guest service lines and they can help arrange everything. That's what I did and we had one of the photographers there. Excellent pics of the entire thing. Since the waiter and manager were in on it too I was also able to slide the waiter my camera set to video and he recorded it. We also put that little clip in a video that played during our reception at the wedding.
It was a little bit of money paying for the pics disney took, but I wouldn't change a thing. It was well worth it, the expressions are great and the prints themselves seem bullet proof.