Well it was really the Mac that brought low-cost desktop publishing to the masses in the late 80's, so you might want to look into an all-in-one system (Plus, Classic, SE, etc) or a LC or Performa pizza-box system. The nice thing about word processing is that it can be done with nearly any level of hardware. As for software, the most popular apps were Word 5.1, ClarisWorks, WordPerfect, and WriteNow. There's a pretty good review of these vintage apps
at System 6 Heaven.
As an example, I'm currently doing a substantial (currently 98 pages) writing project on a 16MHz Mac Color Classic with 10MB RAM and System 7.1. I'm using WriteNow 4 which has all the essential features - styles, spell check, page numbering, etc. It runs pretty fast since large portions of it were coded in assembly, and it's been completely stable - I've been using it for months with zero crashes. I print to a networked HP LaserJet 4+ with a PostScript card, which takes some time but works reliably.
For desktop publishing, you'll probably want a faster system that can handle true color. I'd recommend a 33Mhz 68030 or above with at least 12MB of RAM, more if you plan to use lots of color images. A Quadra system might make sense here - either a Q700 or Q950, or a Q840AV if you've got the budget. Popular apps included PageMaker, QuarkExpress, and FrameMaker, with support from Photoshop, Illustrator, and/or Freehand.