These guys have a plc that can log data:
http://www.controlmicrosystems.com/index.html
I don't know if they will be able to log as much as you are asking, but you can check it out. I have never used them. I have seen a demo earlier this year where if the SCADA and PLC are disconnected the PLC keeps saving variables. Then when the SCADA is plugged back in it receives all of the saved values. I also don't know if this feature will work with your existing SCADA, or if you need to use theirs.