Though it would likely be easier just to hack OJS in this case, I would suggest making this a generic plugin, as it will ease upgrading, and will more easily allow you to share your work with the community (and I think this is a good idea that deserves to be shared!)
Secondly, It would be better to give the plugin its own table, both for database consistency, and as you say, it would require changing a lot of code just to add the ratings/visits as attributes to the published_articles table. Look at the plugins/generic/thesis plugin for an example of a plugin that creates its own table. Also, I would suggest looking at the OJS technical reference (http://pkp.sfu.ca/ojs/OJSTechnicalReference.pdf
) for more information, esp. the chapter on plugin development.
Let us know how it goes and if you need any help...