One of my first projects ten years ago was a similar venture.
Start with a spreadsheet. Capture the three types of requirements: Business/functional(what does it need to do), System(what environment does it need to operate in) and Performance(what are the performance criteria to be successful).
You must define the requirements in such a way that you can objectively rank your current app, and any new app or off the shelf against the requirement.
Its probably better for objectivitity sake that you dont' have the original requirements - you should start this exercise with a blank sheet of paper.
Once you and your users/stakeholders have agreed the list and definitions of requirements, you should determine the must have requirements - which requirements are absolutley positively manadatory. For the rest you should determine a weighting scale(I use 1 to 5, if its a zero drop the requirement all together) and then a scoring scale(again 1 to 5). Your requirements should reflect your ideal product. Then youn need to socre your actual product, and any alternatives against that scale. This should be done in a meeting with lots of opportunity for discussion to ensure that everyones interpretation of the requirements definition is the same.
Hope that helps.
James
