The send mail step can send multilines container elements, however:
You will need to create the element - an attribute of the purchase requisition object (by the way, why aren't you using the standard BUS2105 or BUS2009 object?) and this will require ABAP.
workflow steps descriptions do not send tables nicely, the option is to use a multiline container of a single field structure (for example swf_lines-line) and concatenate the data to it.
"item 1 - material zyx - price 2 USD - amount 50 units"
"item 2 - material mvb - ..."
Another option is to replace the extended workflow BSP, it that case you can sand tables (the BSP has the option to use html tags) however the display in SAP will still be the same, and this again will require ABAP.