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).
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.