What I have done for clients in the past was to use kitchen cabinets. It allowed for a modular setup that you could move the bases around until you came up with a good working layout. There are a couple of cheap programs out there that allow you to do layouts with all sizes of cabinets and get 3-D views and walk-abouts. Basic cabinets are 2 foot squares (these are the easiest to position). You can hang the keyboard tray between two cabinets. You can make tilt tops if you want to display work to a client on a flat surface. Keep the countertop material simple, no patterns or prints.