I am in same situation. This was our solution for our remote and laptop users.
Everyone uses Outlook, No matter what. No express.
Since we have a firewall we needed to set up VPN thru it to allow access to the mail server. If this is not the case for you then don't worry about it.
When we set up Outlook I used the friendly server name instead of a IP. I also set up a LMHOSTS file on the client side that contains an include line. This is so we can make IP changes easily on the server side without a mass deployment. Since their connection will not know what the Mail servers name is, it will look to its host file for resolution.
In Exchange, when you set up a persons mailbox, in the e-mail address tab you can set the defaultsmtp reply to address. You can also "bind" many addresses to the same user. IE, My address is mmorell@vdat.com, and it is set as the reply to address. But in that mailbox I also have set some of my aliases like mike@vdat.com, mike@directionweb.com. Whenever my mailserver receives an e-mail for one of these names it automatically delivers it to my mailbox. But when I respond it defaults back to my reply to address.
There is a way around that but it is for another time.
Other issues that you can address with using Outlook instead of Express is the availabilty of e-mail offline. This is taken care of by the use of Offline folders.
I suggest setting the connection state also to manual so the user has the option of connecting or working offline. (Only for traveling or remote users, not for in house.)
Everything you mentioned can be accomplished using Outlook2000. Throwing express into the mix just makes more work for you.