You will need t get an idea on budget for this project. Also discuss w/ teclient which trees are the most iortant to keep. From the photo it's tough to tell which of large trees are dominant and/or are healthy as to selction of trees to keep and eliminte. I wuld suggest probablya few large boulders (ned skid steer), probably some large rhododendron, mountain laurel, japonica, or other fowering accent shrubs. Get their tastes for shrubs and stick w/ plants whic suit your hardiness zone and site the best.
If you fail to plan ..... you plan to fail.