Where is it going to go? Desk space is precious at the best of times. Do you have a spare desk?
The desk it is on should be at right-angles to the windows so that the sun does not cause problems to the workstation's user by either shining in her eyes from behind the monitor or by reflecting in her eyes off the monitor.
The desk needs to be deep enough for the monitor to sit back far enough to be comfortable to use and for the keyboard to have room to be moved in for the particular user's requirements.
The height of the desk and chair need to be adjustable. Most workstations come with ergonomics guides. Read one.
Can someone use the workstation and a phone at the same time? If the workstation's user is ever going to telephone for assistance, it helps enormously if s/he can work at the machine and talk to the technical assistance at the same time. A new telephone is not always necessary. If there is a socket somewhere in the office just provide it with a long enough lead to reach the machine. When software or hardware are failing this can also provide much-needed comic relief as well!
Do you know how long the warrantee lasts on your machine? Put a note in your diary as soon as it arrives for a month before this time to arrange maintenance for the hardware and software.
Who is going to look after the machine from day to day? Do you have a Departmental Computer Officer? Who is going to do the backups, apply the patches, install new softeware, upgrade the operating system, secure the machine and so on? If the person lumbered with the job is new to it, expect her to take out at least a fortnight getting the swing of the system, and to have to spend at least half a day per week maintaining the machine.
Have you arranged for your library to buy the documentation that your new system administrator might need? If you want it in paper format, it isn't cheap!
Workstations generate huge amounts of hot air. Have you considerd the effect adding one will have on the temperature of the office? Workstations have a maximum working temperature of between 35 and 40 degrees Celsius. If your air conditioning isn't up to it your office can reach these temperatures. (Unix Support has experience of thermal cut outs cutting out in offices whose windows face the sun in summer.) You might also want to consider buying blinds.
Most workstations these days have a fan in them. One workstation on its own doesn't make too much noise, but once you have a few in one office then the noise can distract from work. If you install air conditioning to cope with the hot air generated, consider how much noise that will generate.
Have you added in all the costs of your workstation? Are you going to do your own backups? You'll need a tape drive. What software do you need? Some workstations come with a right to use the operating system but without an actual installed copy. If you can't borrow a copy of the CD you'll have to buy that extra. Are you expecting a C compiler to come with the operating system? Check. Several vendors have unbundled the compiler and you have to pay extra for that. Have you checked that the software you want to run doesn't rely on extra RAM (which will cost over and above the base price the salesman showed you.) Have you really thought about how large a screen (both in physical size and number of pixels) you want to work with? Chances are that the price you have seen quoted is for the minimal screen.
Don't forget the recurrent maintenance cost on all these components. (Yes, even software carries a recurrent cost.)
What software do you need? What software do you want? Select your workstation so that the commercial software you want runs on it. Do you have practice in compiling up all the public domain software that you rely on? Of the software you currently use on other systems, are you even aware of what software is public domain and what is vendor supplied?
Does some of the software you want rely on expensive extras? These might include extra, non-default software packages from the vendor or simply more RAM than comes by default.
Does your Department have a policy on what sort of machines to buy? Are you going to have the only machine of its type in your Department?