My thinking is to purchase a computer with a Pentium-based processor that is expandable, in terms of both ram and hard drive. Notice the mass of paper spread out on their desks. Watch the interruptions that force them to shift from one task to another. You will need a system that allows people to change gears as quickly and flawlessly as possible. I favor the Pentium II microprocessor, and recommend at least 64 megabytes of ram.
Some programs require at least that much.
Also, ram is relatively cheap. When you ask about prices, you will notice that at some point on the scale, acquiring the next faster processor forces a large jump in the overall price of the computer. Unless you have a specialized need, you should buy the processor at the sweet spot, the level just before the large price increase. Finally, purchase two or three times as much hard disk space as you think you will need. That also has a sweet spot.