Besides the choices listed by Robert in his answer, which probably won't work in the latest versions of WinEdt (version 6 is quite obsolete now and has not full support for Unicode), WinEdt has a built-in completion feature.
Execute the command 'Complete Word' (Ctrl+Enter) in the Tools menu and WinEdt will display a dialog with a list of possible completions. For example, if you press that shortcut after having typed "yest" you will get:
If there is only one possible completion, this is automatically inserted without displaying a dialog. If, on the other hand, there are no possible completions for this word (indicating that it is misspelled), WinEdt displays the Spelling Suggestions dialog:
The same happens if there are more than 255 possible completions (indicating that the action has been initiated on a string that is too short to merit accurate completion).
The list of possible completions is compiled from the WinEdt dictionaries that are enabled for that document's mode (and submodes). Specialized dictionaries are made of LaTeX commands, WinEdt macros, and other keywords enabled for a particular mode.
Dictionaries for languages different than English are available in the Dictionaries section of the WinEdt's community site. The macro InstallDict simplifies the installation of new dictionaries and is useful for WinEdt newbies.
Note, finally, that you can also add your own words to a custom dictionary and then use them for completion.
This can be done by pressing 'Add' in the above dialog (the latter).
It's not a proper "dynamic completion", but it does what you were looking for.
If you are also interested in "environments completion", see this answer of mine.