USB Underwood No.5 Typewritter
If you are going to modify an antique typewritter, an Underwood No.5 is a good choice. They were incredibly popular and you can still find them pretty cheap, and sleep well knowing that you are not modifying a rare museum piece. I have modified this one with a kit that allows it to function as a USB keyboard for any computer, AND it still retains all of it's original paper typing functions. Fun fact: My iPad fits right into the carraige like it was made to go there and can accept typing unput through a USB adapter.