I recently purchased the Nikon D3200 which I would like to use as a second camera on shoots. But there will be times it is used as the sole camera on shoots. It comes with a built in mic that filters audio decently. I know I need a better lens. What parts would you suggest I get to accompany the body to produce the highest quality possible for say documentary shoots?
I would suggest Rich Harrington's (and co authors) book, from Still to Motion as a primer. If you are studied and a film maker great, but, for HD enabled DSLR's, even for a pro, that book is a measure of intent and focus which covers soup to nuts, the attendant 'issues' that you may need to choose from, gear wise.
Specific to your question, dual source audio may be of some importance, a Zoom or other audio capture interface etc. to isolate a clean audio signal but, there are many issues besides gear you might need to examine regarding how Nikon captures video.