Hi Brian,
Sorry for the late reply. There certainly is a way to attach documents. I would recommend copying and taking a look at the CRM_EditCase screen. You could essentially copy most of the actions on that screen and place them in your portal screen for cases. You would be creating your own Documents section (like in the Edit case screen) for your portal screen. You probably won't need all of the actions for that screen copied as that was designed more for the Method user, but most would apply. Like that screen, you can include the grid, the document and save buttons.
As for the emails, the stock screen will input the portal administrator into the AssignedTo field. This user's email is then used as the From Email address for the email to the contact. For the email to staff, the Send Email action is set to the portal administrator, so again that user's email address will be used for the From Email. If you wish to change this, there are a couple of way to do it. You could change the Assigned To user, which will change where the From Email address for the contact email comes from. This can be found on action #1 for the OnScreenLoad. You could also change the email in the Send Email actions on the submit button. Keep in mind there are two Send Email actions. One for the contact, and one for the staff.
For the tabs, the stock Contact Portal is based on the Contacts_Portal tab. If you edit the portal, you can change which tab the portal will point to. Also, if you want to adjust which screens will show, you can go into Customize > Tabs and adjust the tablinks for the tab.
Hope this helps,
- Adam