Out of curiosity, what OSS options are out there that offer the breadth of functionally that either SAP or Salesforce do?
I'm not familiar with either SAP or Salesforce feature sets but if you are seriously considering them you should look at Odoo first. You could use that, hire half a dozen full-time programmers to tweak it and still come out ahead. It is also more likely to be useful out of the box than SAP.
If that's not open-sourcey enough there is also Tryton which was forked from an early version of Odoo. Not as many features, but some technical improvements. Odoo modules should be fairly easy to port if they have the right licence.