I support blackberries at my job and was around during the actuly user rollout. A seperate division actually handles the blackberry backend, but here is what my company uses:
The 957 (palm sized)
http://www.blackberry.net/products/r...57/index.shtml
and the 950 (pager sized)
http://www.blackberry.net/products/r...50/index.shtml
Most of our users prefer the 957 (palm sized) ones because they have a larger screen and have an internal rechargeable battery, the 950 (being smaller) is not as popular and you need to change the battery (AA sized) every month.
As far as the wireless provider, here in the US you will pretty much be stuck with Cingular.
As for personal recommendations, I would suggest that you look into one of the newer models with triband (worldwide) support.
http://www.blackberry.net/products/h...ds/index.shtml
Depending on what your company will pay for/what the execs in your company want, the phone feature is something that most of our users are interested in (along with the worldwides support). I think that these models are still in evaluation at my company though.
The BES is the way to go because the desktop redirector requires that the user's PC be on for their e-mail to be forwarded.
Some tips are that you should be using the latest blackberry software v2.1 or later (because it fixes an issue with syncing the e-mails on the blackberry with outlook/exchange, the older versions didn't sync the e-mail).
When installing the software on the users machine, you need to have the blackberry hooked up and in the cradle, then after the software is loaded and the desktop manager is running, you ned to go into the connection settings to detect the com port (not sure if this is needed on USB versions). IF the blackberry is NOT found, enter in the password on the blackberry and try again. After you do that then you need to go into the intellisync options and configure the calender/contacts/etc so that it syncs with the e-mail program you are using (outlook/notes/etc).
As you can tell I have done a lot of these installs at my job, but I hope that it helps you with the questions that you had.