What method will your client be using to access the keys and mouse? I have used typing aides, placed pencils into u-cuffs or instructed clients in use of dragon naturally speaking software for those who use computers very frequently.
We use Guldmann overhead ceiling lift systems where I work. With the correct sling size the clients report they are comfortable and they are easy for staff to use. I believe the weight capacity for the ones we have is 550#.
I have also used a head mouse, the mouse control options with the dragon nautrally speaking software and a chin control mouse. Other good foot control options are http://www.fentek-ind.com/nh-mouse.htm