Freejung already covered things some, but as noted the user table is already present and available for your use. Plus it gives you the ability to allow user logins down the road should you wish to add that capability. Allowing the user to see what they have in their possession for example.
In regards to Evo to Revo differences, the permission structure is totally re-hauled and more like active directory on windows servers. It's also easier to integrate with external systems, for example we got a basic Google Login snippet working in under an hour, which would be much harder under Evolution.
In regards to building the application, if this is going to scale any, how you setup the tables is just as important as the code driving the system. If you don't know, I would highly recommend asking, or as noted above hiring out the build. I know we have several custom database driven tools built on Revolution, it's not super difficult to accomplish if you know what you are doing.
I hope that helps some. If you have questions, you know where to find me