Writing a Software Procurment Proposal

I am re-asking this question in place of another post that didn't get specific, or answered. I have been given the task of writing a proposal to buy the new Adobe CS3 package for our marketing/design department. I have all of the information I need (I think), and now I have to justify the cost of buying the software. It shouldn't be too difficult considering our current software is 3 versions out of date, but I have never written a professional software procurment proposal before.