I would look for Office 2004, and not any earlier version, mainly as MS has a free utility that allows you to read docx etc. files in it. That alone, apart from numerous interface enhancements, makes it worth the trouble to find and the small extra expense to purchase. Office X (2001 in an earlier post) is not what you should go for if what you want is maximum compatibility, though it is a much more Mac-like version of Office in its "Classic" (as opposed to OSX) form.
Office 2008 will even read Word 5.1 files very nicely, as will Office 2004 (rather more easily than Office 2008, in fact); either of the two, or indeed Office 2001/ Office X, will read your Office 98 files seamlessly.
Say what you like about MicroSoft; in their Office products, they have done us a huge service by maintaining excellent levels of compatibility between old and new versions of the software, as well as between the versions running on Windows and the different versions of the MacOS. For that reason alone the MS Office product (for anyone who works in an office or in education etc.) is indispensable.
I do not especially admire the way Word, for example, works, but I could not function on the Mac without it.
And Bill Gates is a saint by comparison with His Holiness
, even though the latter at times assumes the aura of the ascetic.