No matter what your word processors are going to be interchangeable if you're just using microsoft word (and if you don't have a full copy a quick trip to our pirate bay friends should take care of that) however seeing as you said your mother is a writer she should have a CD for word laying around somewhere.
If you can't get microsoft word and are against pirating software, you can just go ahead and get a free program that mimics a word format, here's a list of some of the popular ones off of wikipedia.
http://en.wikipedia.org/wiki/List_of_word_processors
Personally, I always used Abiword and Openoffice (I preferred open office because it looks identical to microsoft word making it more easy to relate with)
Other than that, as far as a good computer is concerned everyone's going to tell you something different. As a nerd, I always hated Dell laptops because they would always overheat on me, the only good laptops dell put out were their alienware gaming laptops, so a good computer for your mom runs in what your budget is. Personally, the only laptops that I get are HP and Mac's. I ran my HP to the ground after about 5 years of owning it and am still running my Mac from 2 years, however a Mac is probably a little out of budget for the christmas fund lol.
So really you should just avoid getting her a cheap kind of bargain brand name laptop (like asus or acer, formerly gateway), because every one of those that I've owned or had has just taken a dump in under two years, which nobody likes.
Hope that helps you out.