Here are the answers to your questions:
1) The system is currently mainframe based. There is a desire to move it to the web.
2) All users **are** required to login. Security is of utmost importance since we are talking about large amounts of money. All sessions are secured. I don't think one could say they are "monitored", but they will time out after a set period of time.
3) The users will have different points of access probably about 20,000 at least -- from all over the United States -- but not the world.
4) Except for maybe a small logo or something similar, there will be no images or ads. This will be basically a system to transact normal daily financial business with existing customers -- not a shopping or "for sale/buy here" type of site with engaging graphics. The users would be mostly clerical folks using the system to do their jobs -- but now doing so via a web browser over the internet instead of a dedicated machine with custom software running thru a secure private link.
5) Accounting is needed both at the global/summary level, the customer level and down to the individual transaction detail level.
6) By "hoping to champion open source", I mean that I strive to see open source solutions at least considered and hopefully implemented when they make technical sense for part or all of a project. As I mentioned in my reply to Don above, in the large Midwestern companies where I typically work, open source isn't even on the radar. I would expect that if open source solutions were implemented, then enhancements would indeed make it back into the community. I try and do what I can. However, without knowing what the upper limits of what open source software is capable of -- take Postgres or AOLserver for instance -- it's not possible to present a solid proposal for consideration for the scale of projects I've been involved with the last few years.
Thank you very much,