You'll want to look into custom hardware, like FPGA's or even go into the yet to be released ASIC based products.
FPGA's and ASIC are custom chips, that are very energy efficient when compared to Graphics cards as normally found in most computers.
These custom pieces of kit vary in how hard they are to setup, but very doable and stable for any techy person and can be managed remotely if necessary.
https://bitcointalk.org/index.php?board=76.0The talk about these is all in here. Some of the current FPGA's on the market are more out of the box than others, but I won't really call Bitcoin as a whole out-of-the-box in general.
Enterpoint is popular in Europe since they are based in England. BFL is more popular in the USA since they are based there. Big generalisation, but shipping, duty and taxes on these is not cheap when buying alot of them, so with them both being priced per performance rates about the same it helps to buy local as such. At least that was my reason for going for enterpoint initially.
I do not know the other FPGA's that well I'm afraid. Their is a few ASIC products that are suppose to be released at the end of this year, but their is much speculation that they might not achieve this. Their is the BFL and OpenASIC project that are doing those ASIC based products, which will be even more powerful than FPGA's. Again much speculation about that.
A large scale (as mentioned already) Graphics processor (GPU) based operation is not that efficient energy wise any more, they are often 5-10x more expensive to run unless you have an alternative use for them. It can of course be a little better on ease of use, but the future of GPU mining is probably behind us, unless you make a very large GPU farm and very cheap electricity.
No company right now sells their products all setup to run as you describe, it still requires you to connect up to your computer and do much of the configuration yourself. They often will help with thing you need of course or at least the community will often help answer any queries.
However right now their is a big separation between Mining hardware, Mining Software and Mining pools, so an all in one solution is unlikely to occur right now. This allows choice, which I believe to be a good thing, it has allowed the best to thrive.
If your computers in the office would end up actually using high-end graphics cards as per normal work, you could just use a bunch of ati 7970's, otherwise go with FPGA's or ASIC's and just plug them into a spare computer.
Sorry that ended up a lot longer than I had planned.