First off ... since Badfish28 never responded back to this thread ... that he started... hopefully all is well and good with his major project undertaking.
Now for those who might just be in his shoes today...here is my two cents on such a implementation.
The shear scope of his project begged 1st of all that a VAN be selected .... Advatis, Ordernet, harbinger.net, GEIS they are all good at what they provide.
I have been an EDI consultant for the past 10 years and most companies that have embraced EDI have a backlog of projects and trading partners. Web based may sound good to you ... but how does it play with the 100's of potential TP's mentioned? I can almost hear their groans ... and then placing your web based project on their back burners.
It can be assumed that most of Badfish's potential trading partners utilize traditional EDI. There is a reason for this in that each TP can batch up there invoices, purchase orders, ASN's, etc and send them off to the VAN and not have to worry whether their TP has a particular flavor of software. Because they have gone "live" it is assumed that their TP is now ready, willing, and able to receive those documents from the VAN. This is usually done at a shared cost.
A bigger "push" in the real world of today than AS2 (excuse me WalMart and the few others who are shoving this down folks throats) is to utilize IP technology to FTP documents via the internet, or via VAN. Files are imported and exported into a tradional EDI translator, and then sent on their way via FTP. This essentially eliminates most of the cost related to VAN charges.
The ability to go this way however is not a slam dunk as the way in which one exchanges documents with their TP is a negotiated method, hence Trading Partner Agreements.
The sheer scope of the described project begged for a batched approach. Many web based solutions are for those who have a limited budget and number of TP's. One does not want to have a room full of people interactively going out to various web sites to retrieve documents. Automation is essential. 3000 vendors and over 300 customers are a lot of transactions folks!