The "ility" side of your requirements will probably help you get to an architecture: availability, scalability, usability, portability, reliability, compatability, and so on. Sounds like you've already gotten as far as J2EE. That still leaves more architecture choices ... plain
servlets?
Struts? Custom tag libs? Spring? Hibernate? Your skills and budget and timeline will influence all of those.
Early on I'd try to pick a candidate architecture, take two or three of your best people and implement one business function from end-to-end, browser to database or whatever. Chances are good that once you get some basic issues under control most new business functions will be more of the same: accept a request, read some data, format and display it, accept updates, apply to database, etc. Then you can pile on the whole team. In my life we get a new back-end or external partner every week, so the interfaces to other systems never ends, but you might get lucky with a very stable architecture right off the bat.
Leaping from a requirements document to an object model is not easy the first, um, thousand times. There are whole books on OOAD. Do you have any good ones yet?