If you have any special needs, be sure to bring to the attention of your travel agent. The newer ships have many cabins which are designed to accommodate a wheelchair and they are up to the current standards. Older ships may only have a one or two cabins - and may not be up to standards. Some ships do not have any cabins for wheelchairs. Renaissance, for example, even though they have very new ships, they cannot accommodate wheelchairs in any of the cabins.
Many cruise ships have no cabins for singles, but there are some that do - still expensive, however.
If you can't find somebody that will go cruising with you, some cruise lines have a plan where they will find you somebody. If not, you get the cabin to yourself. Not all cruise lines offer this. If you book a cruise this way, you don't know what your cabin will be. Some cruises can't be booked this way, especially the longer cruises.