Hi frostyshield,
The online documentation for MMO Kit can be found here
If you don’t want the kit’s registration/login, the auto-updater and the gameplay examples, you probably don’t need to purchase it - the default UE4 networking could be enough for your project.
Personally, I’m a fan of Steam and I’m against ‘reinventing the wheel’, but not all Steam features can be used right away - for example you can use Steam’s downloader and achievements only once your game passes Greenlight (or if you have another game on Steam already).