I have an Atari 1200XL, a Rana 1000 disk drive, and am using a TV set as a monitor. Would I need to expand the text field to 80 columns to accommodate a letter-quality printer?
This isn't necessary. An 80-column video adapter board is nice to have when you're using a word processor to prepare a document because the screen can show how the document will appear on paper. It's not required, however, because the word process sor allows you to specify any width for printing—including 80 or even 132 columns (if your software and printer can handle this). The size and format of the video display does not limit your choice of a printer.
You should also be aware that most TV sets cannot adequately display 80 characters per line; the characters will usually be much too fuzzy to read. You would need to buy a monochrome computer monitor. In addition, we haven't heard of any 80-column adapters for the 1200X1, and it's not likely that any will be sold. Unlike other Atari computers, including the 600XL and 800XL, the 1200XL has no expansion slot.