1996 Mercury Grand Marquis needs a new power steering unit on a scale of 1 to 5 5 being very difficult is this an easy job or a very difficult job?

Depends on a couple of things. First, are you any good at mechanics? Two, do you have the tools and a place to work. If yes, changing the steering rac is probably a 2 to 2 1/2 job. Jack up car and secure on jack stands. Unbolt the steering column, remove the rac bolts and tie rod ends, remove the rac. Assemble in reverse order. No big deal. However, if you ain't good at that sort of thing...