Yeah ... 26" models are not very portable and carrying it into the office might be more difficult that you think. I would stick with the 20" bikes.

You want to determine your price point before doing much more work.

You should also consider how much you like to tinker.

From what you describe, I would just make sure that it can fit fat tires. Assuming that you are not spending a ton of bucks, my preference is to avoid suspension bikes; but others here feel differently.