moving a server & adding a PO

Currently the system consists 2 Servers
OES11 + GW2012

Due to reorganization we are moving a whole department to another site
the site will be linked by Site-Site VPN over an ADSL link

Server 1 - user files
Server 2 - groupwise inc webaccess & gwia - single post office

So my thoughts are to add a 3rd server & add a 2nd PO
move users concerned to new PO

Server 3 will have PO, MTA & Webaccess installed

There is a GW Document library being used by most of the users - so will need to have a library for each PO

Mobiles are setup as IMAP - I have 2 GWIA's running on Server 2 (2nd GWIA for out bound only for mobiles)

Questions:
First of is adding a 2nd PO the answer or do I need to consider anything else ....are there any "gottchas"
Would users at both PO be able to use mobiles via GWIA on Server 2
I'm NOT sure how the documents will move...Document Management - when I move users from one PO to another would the documents move to the PO ?

Folder Shares - what happens - would sharing still be valid?
Documents - what happens to documents that are currently shared ?

Anyone done this before? - any advice appreciated