If you are going to be doing a lot of writing (typing) on it then you will want to test the keyboard before buying anything to see how you like it. You'll also want to view the screen and make sure you're happy with how well it displays text. For those reasons, I recommend that you don't buy from Ebay or other mail-order places without the ability for any hands on demo.
Word processing is not a very demanding task so having a lot of memory or a fast processor won't help much. Keyboard, screen and storage (disk space) are the 3 factors I would be considering, not price.
In order to save some money, you could use the open source (free) LibreOffice or OpenOffice office suites both of which are capable of reading and writing MS Word compatible DOC files.