Import of purchase orders and amendments to POs

We are looking at importing POs and amendments to POs for one of our other systems.  These will then need to be matched to invoices (also imported from the same system)

I've read about the PO Import and understand that there is a sustantial part that we will need to code.

I'm trying to establish what attributes we need to import and if the amendments from the feeder system can be imported (and how)

Has anybody done this ?

Pam