As mixture says, there is no reason why you should not install either the Office for Mac suite or use VMware so that you can run the Mac as a Windows machine. However, neither options are free and a free alternative is OpenOffice or its derivative. I use OpenOffice on my Mac and it does pretty well everything that Microsoft Office can do. It can read all the usual Microsoft file formats such as doc, ppt, xls and, although it has its own format, it easily exports to the Microsoft ones.
Going for the cheapest Mac that will run the latest operating system might turn out to be a false economy as Apple introduce a new operating system every 18 months or less, and each one is more demanding in terms of system specifications than the last. A Mac that just about runs today's operating system might well be incapable of doing it for tomorrow's. For the last few interations of Mac operating system, it has been free to upgrade so, if your computer cannot handle the new one, you are denying yourself the opportunity of upgrading at no cost. A major benefit of owning a Mac and an iPhone (and/or an iPad) is the ability to syncronise them so that, for example, calendars, notes, reminders, photos and contacts are automatically updated across each device. However, this may not be possible if the standard of operating systems across the devices diverges too much.