Assuming this is the Jade then wheel chair access is not a problem to most areas(we travelled with a wheel chair user this year).
Some areas are not 100% convenient all the time like the buffet at peak times which can get packed
The main features of the cabins are wider doors and wheel in showers these are the same cost as the same grade regular cabin and are much bigger typicaly around 1.5times the size
There are 4 berth cabins, check on
http://www.ncl.com/nclweb/fleet/deckplan.html?shipCode=JADE (I will do this now) you need to look for a black square block(disabled) and a +(3/4) or a star(3/4/5) passengers.
The 8 disabled G grades on the Jade are a good comprimise on price and location having handy access to the blue lagoon for 24hr snacks, 2 windows since they are sideways cabins and close to the aft elevator section for easy access to the rest of the ship
The 2 deck 11 aft facing AF disabled cabins are great with a good balcony, the other balcony ones are located on the aft elevator section so good location for getting about.
One problem on location is that wheelchairs and the cabin steward trollies can clash in the mornings so a cabin near the elevators is something to think about to avoid this.
If you have any specific questions I can try to help.
For NCL you may find it cheaper booking through the US for some dates.