How can I start typing a word, and have Microsoft Word offer me suggestions so I don't need to finish typing?
Is there an addon I can buy or download that would do this for me?
There is Phrase Express (freeware).
Organize frequently used text snippets.
Expand abbreviations as you type.
Launch programs with text shortcuts.
Auto-complete repetitive phrases.
Quick access to the Windows Clipboard History.
Correct spelling mistakes in any application.
You can also create your own suggestions to auto-complete...Works on Windows, cross-software...
There is not AutoComplete in built into Word such as you'll find in a web browser or the like simply because the possible words you'll type into a Word Processor is much, much larger than the list of possible words you'll enter into a browser address bar.
Microsoft did re-enable a system called QuickParts or QuickText that allow you to enter a pre-defined code or set of keys which Word will then replace with a longer phrase that you had configured previously.
You can find more information on this built-in system here and here.