I bought a 521 for my wife through HSN for $100. Called TMO and got the $30/month, unlimited data, unlimited text, 100 minutes of talk plan. If she needs more talk time she can add money to her...
As you indicate, you don't spend your day in an office. Outlook is indispensable. I much prefer it to a web-based interface.
I would pull the trigger on an RT device if they offer the full...
From the Dell and Apple company websites:
Dell XPS 10 - 1.4 pounds
iPad - 1.44 pounds
Just upgraded a user from XP to Windows 7 a couple of months ago. She has not stopped complaining about awful it is. The issue is, of course, that she has to learn something new.