First off, I'd recommend a migration vs a new build. Unless you truly suspect problems with AD, the impact on end-usersis much less significant with a migration. New builds always introduce unexpected and unwelcome security permissions issues as old SIDs stick around on files in unexpected ways.
Even if you want to rename your domain, you can migrate the user accounts and data via ADMT.
http://technet.microsoft.com/en-us/library/cc974332(WS.10).aspx
As far as Exchange goes, since it sounds like you'll be doing a hardware refresh as well, that makes things easier as both machines can exist at the same time. MS actually has a lot of documentation on upgrading your infrastructure to Exchange 2010 from 2003.
http://technet.microsoft.com/en-us/library/dd638187.aspx
In short, with only 100 users, this should be a pretty straightforward migration. As every environment is unique, I obviously cannot gaurantee that, but I saw nothing in your post to make me overly concerned. Just a matter of planning, documenting, and then executing.
