First of all, standardise on a single distro so that the 99% of people who aren't Linux gurus can at least share solutions to problems. It's quite common to have hardware that doesn't work in some way, e.g printers, sound or graphics cards (3D performance), and it will be disastrous if everyone has different distros.
Secondly, I'd recommend Linux Mint - either the Ubuntu or Debian based version. It has a lot of simple but helpful changes for new users of Linux, but the Ubuntu/Debian base means an enormous amount of software is available. I wouldn't recommend Ubuntu these days, as it has too many regressions from release to release, and things that just don't work (had to abandon an Ubuntu 10.04 LTS installation as it froze every day or two for months, probably due to Intel drivers.)
The switch to kernel mode setting (KMS) for graphics cards in the last few distro versions is critical - in some cases this has really reduced reliability a lot, so I'd recommend you research this a lot... I ended up using an old ATI 9250 graphics card to be sure that Ubuntu (or Mint-based Ubuntu) worked properly - however doing this for a whole company would be painful. This is important given the popularity of Intel GPUs on business PCs and the crapness of Intel drivers post-KMS, but perhaps some research will show this is a non-issue with the latest kernels and X11.
The switch of most distros to GNOME 3 and/or Unity is also a big problem - these desktop UIs are very immature and simply don't work well for the sort of desktop usage many people are used to. Unity in particular is a research project that should have been left to mature for 5 years, not pushed into a long term support release - this is why a big chunk of Ubuntu users are switching to Mint or other distros.
Mint has a sane strategy for GNOME 3 which involves recreating the GNOME 2 UI on a GNOME 3 base (Cinnamon project, aka MGSE), as well as letting your retain GNOME 2 if you want (MATE, not yet mature). Most importantly, Mint as a project listens to its users a great deal, so it is less likely to take decisions that screw up the user experience (e.g. Unity.)