I used to work for an office supply company so that is where I would look for a chair. I know I would want one with wheels so knowing that I would suggest 5 wheels v/s 4 so it does not tip over. Next suggestion is to think about how long you will be sitting in the chair. Make sure it has the cushion that is comfortable. Then look at something that will adjust up and down to a height that exceeds what you think you will need so you have options to raise and lower within your comfort zone. It is also nice if you can adjust the tilt of the seat a little which will change your weight distribution slightly and make a difference in how long you feel comfortable at the machine. The best way to shop for a chair is to go and test them out. I have been known to sit in several then drag the best three to a display desk to see how the fit when not out on the open floor.
Good Luck!