To start with, if they wish you to prepare the Partnership returns, you need copies of AT LEAST one most current prior return. (I would probably ask 3) Having more than one will let you get a feel for the businesses and whether they are fairly consistent from year to year, or whether the years vary greatly.

The more you understand the better off you are. Also it sounds like some of the businesses are at least related, or possibly one own's one or more of the others. 

If you can gain an understanding of why the operation is structured this way, that will help as well.