I haven't had any issues with 2000 -2003. I had publisher and access 2000 installed with the rest of office 2003. I would say to try it. If it doesn't work, then you should be able to uninstall 2007. Express and regular outlook should be able to function together. One of them will be set for the default mail client, so just decide which one you want to open if you click on a mailto: link.

Word and Excel should be backwards compatible with anything you have from 97. You can edit and even save in that format. Depending on cost, etc, I would just upgrade everything at once. You should be able to re-install if you have issues.