Volume/Transaction Estimates and Hardware Sizing

We are at the beginning of an iSupplier Portal implementation.  We are looking to put together a sizing estimate so we build production hardware large enough for the transactional volume in the first 1-2 years of production (with scaling capabilities to grow down the line).

Has anyone put together a transactional volume estimate for iSupplier based on number of suppliers, number of POs and/or number of invoices?

Something like 2% of the supplier database would log in and make address/contact information updates.

There would be an average of 1.5 iSupplier views of PO data for each PO created.

There would be an average of 3 iSupplier views/transactions per invoice created.