I have a Computer Engineering degree from Carnegie Mellon U, and I've worked for 15 years as a designer and developer. I know design patterns very well, and I know UML well, if you want your diagrams done that way. In any case, this doesn't sound particularly complex, and I think I could come up with a good architecture within a few hours. I've designed 20-30 small systems, and they've all turned out well in the end. And, I won't ask for any pay until you're satisfied with what I've done.