Configure your own...
Buy an off-the-shelf model and protect it with a ruggedized case. OtterBox is one of the most popular manufacturers of such, though you'll need to look for a higher-end provider if you need waterproof and crush-proof cases.
To answer your other questions:
1.) Yes, an unlocked smartphone can function as a PDA. In fact, the smartphone need not even be unlocked in most cases - just purchased off-contract. (Unlocking would let you use it on a competitor's cell network.)
2.) Windows Phone 7 phones have native support for Microsoft Office documents. Android, iOS, et cetera devices generally require purchase of third-party software in the $30+ range.
Hope this helps,