While there is some solid advice here and would encourage you to look into the options provided, I wanted to give you something a little more immediate, based on having dealt with two major ERP replacements and supporting a third, in my career. Essentially, the role of the BA is to be curious, to ask questions, and gain understanding. In my experience customers will try to prescribe a solution based on their own understanding of the technology or because they have always done something in a particular way, but more often than not they are not best situated to provide the solution, due to their lack of understanding regarding all of the factors involved. For this reason, it is so important to start with 'why'. Understand the business problem or requirement that the solution needs to address, without reference to technology, process or user experience (user interface, report layouts, etc.), and really understand the value that achieving that outcome will have to the business. Doing so will both aid in the prioritization and may potentially eliminate some requirements altogether if the value just won't be realized.