These are really a set of questions, but as a general recommendation, I would say that you shouldn't jump onto anything. You should investigate the opportunities and test your assumptions. Trying Ubuntu is very easy to do without any consequences. You can just put it onto a memory stick, and then you can boot it. It all takes just a few minutes. Familiarize yourself with it first, and then test it on your users.
In my experience, the less experienced users will have the least difficulties switching operating systems. After all, most of the concepts are very similar. The more experienced you are, the more the differences counts. I wouldn't expect the user experiences of Windows and Ubuntu to be a large obstacle to your users.
But from your description, your company seems very locked down into environments that themselves cannot legally be used in other environments than Windows. I would begin with replacing applications one by one, making sure that the replacements are available on many platforms. For instance, if you use Outlook for email, then you might want to see if there's a chance you could switch to Thunderbird, since it is available for both Ubuntu, Windows and OS X. Once you know that all applications are available on different platforms, then you can switch the operating system itself, and the users won't notice much of a difference.
Of course, your primary goal is to get your stuff done. It should also be a goal for you to increase your company's flexibility by reducing the platform dependency. Because there are lots of benefits of using free software. You might want to start using other types of devices, like tablets, and having a clear focus on platform independence will help in that regard as well. But don't rush into things. You should define your long-term goals and only replace things when you've tested to make sure that the replacements will get your stuff done.
You probably won't need to hire specialists. However, Canonical offers support services at reasonable rates, and they have a lot of experience with professional migrations. I would certainly recommend that you get in touch with them and see what they have to offer. It might make things very much easier for you.
In any case, I would wait until Ubuntu 12.04LTS is released. It'll be supported for five years and it's a significant upgrade from 10.04LTS. And you should stick to LTS releases, which are upgraded every two years.