Do you have to carry a lot of stuff? Do you have to lug their computer back to your place? If so, get a rack and panniers or even a trailer. If not just use a backpack. 3-6miles isn't much so almost any bike could work.

Since this is work related, get the most flat resistant tires you can afford. You'll loss customers if you show up late because of a flat. I recommend specialized armadillo 26 x 1.5 tires. Good peace of mind knowing you ride one of the most flat resistant tires on the market. It may seem expensive but it is worth it in the long run since you won't have to replace tubes as often (almost never, these tires are tough)