You will for sure want to check out HydroGuy's thread on World of Color:
http://www.disboards.com/showthread.php?p=37986447
There is a viewing deck at the GCH, but it is not ideal and would be difficult to see. It is a side view with lots of obstructions. You will barely see the projections at all.
The thread above does a good job of hashing out the benefit of the World of Color dining packages. Bottom line, if you were going to have a nice sit down dinner anyway OR you won't have time to obtain a FP during the day, the WOC dining may be a good plan.
The thread highlighted above has a chart that shows where the handicapped sections are. They are in the back, but on a raised portion. There are
benches there for those who need them, and also spots where you can roll up a wheelchair to a railing with a clear view. You absolutely need a FP to enter these areas even with a wheelchair. I have seen the show from the blue handicapped and yellow handicapped. Both are excellent views. The handicapped section of the preferred dining area is smaller and is taped off on the ground, not separated with ropes. It is closer to the show, however.
As a note, the queuing system when you have a wheelchair is a bit different. The CM should direct you, but you will have a separate waiting area in front of your color and be allowed to enter first. Once people are settled, the rest of the color is allowed in.
I hope this answers your questions and that you have a great visit!