Alcove and Rik have the knowledge you need.
Shoot for the highest sound quality you can, shoot it on whatever camera you can afford/borrow/whatever.
Deep and Broad access to your subject material is more important than any technical consideration in a documentary. Your material and the content-related (as opposed to technical) are your bread and butter. Highest quality audio you can get is the toaster that makes it all come together.
Might be odd for people to hear me say this, but in camera terms you just want anything you can get that will give you manual exposure, white balance, and focus, and will allow you to operate it for long stretches of time without issues like overheating, being too cumbersome to operate hand-held for a full day, and so on.
If you can borrow some lighting and such from your cousin, all the better as that puts you ahead of the game for interviews - though my camera comments above are based on the assumption you'll be shooting more material to go with the interviews.