I beleave you can use a separate comments module, or if it is not that big you can just create a component with a controller and a few views that you can reference in your modules.
The comments on each section would work the same so thats why I would prefer to just have them in one general place then I can add a DB column so you know what section they belong to.
the portlet looks more like a widget that you can use to display comments
on the page i was reading one of the portlets to be built was "recent comments"
I need something that will not only display comments but also allow users to post comments
Well… a widget that :shows the comments AND a form to create a new one? (both in the widget ofcourse, sorry for my english, edited this post like 5 times so it would be clear, but I gave up and added this note )