I have looked in the edition comparison table and I can say that I don't need any of the features exclusive to Professional.
Are you sure about that? You might not think you need them, but your IT staff is likely adamant that you do.
The main sticking point is usually joining the machine to the domain. IT staff use this to do things like set up your user account, control access to file shares (and possibly an exchange mailbox), deploy certain kinds of software to the machine, and generally get their job done. A machine not joined to the domain will require a lot more work for them to keep up with than one that is joined.
While this isn't really a problem on one machine (you can set up work-arounds for most things) it's definitely a problem if there are lots of machines like this; it will cripple their ability to keep systems functioning efficiently. This is the kind of thing where making exceptions creates a slippery slope; once they allow you to get away with it there's suddenly 20 computers to support that aren't connected and all need special attention. Now those extra few dollars you were so happy to save are lost 10 times over because you're paying for an additional support tech.
Additionally, that list you linked to is far from exhaustive. For example, it's missing remote desktop connections, and that's another big one for IT departments.
All that said, there's no legal reason you can't do this.
As a final note, do you really need that fancy Core i7? Where I'm at it would definitely be considered a luxury item and require a pretty solid business case to justify. Anyway, if you really want a fast machine have them spend the money on a quality solid state drive. It's likely to matter more to performance than the fast processor, as even modest i3 is likely to have cycles to spare.