Hi Angela - there's a lot of variables at play that would impact this analysis.
1. Do you plan on doing collaborative requisitioning with your vendors? If so, the collaborative requisitioning model Ariba has in place uses the Ariba Network for collaboration requests notifications and the punch-in process to your Buyer instance.
2. Whether you're on the cloud based On-Demand version versus On-Premise will likely impact what you can and cannot do (meaning you have a lot more options On-Premise).
I agree with Ken that there's a tremendous amount of value in leveraging the solution Ariba has developed via Ariba Buyer and Ariba Network integration for the collaboration phase and the PO/Invoice. However with 3000 vendors to deal with, I hear your vendor enablement pain.
As for the T&E tie-in, are you referring to the Consultant Time Sheet Entry feature which is kind of like partial invoices, or the Ariba On-Premise T&E module? I'm not aware of any out-of-the-box solution with T&E module.
In terms of working with non-Ariba Network suppliers, you'd have to consider what you're going to do about the collaboration component. It likely means this phase of the request may happen outside of the system. I've seen clients set up the contracts for temp labor and other services, and then leverage non-Ariba-PO Invoice against contract to capture the spend. This was done with the On-Premise software where there's a lot of control over how you integrate non-ariba POs. You could also look at handling the negotiations outside of the system, then have your requesters enter non-collaborative requisitions with an order method like email/fax. Again, not sure you can do that with the cloud version.