Ms Access Offline data Sync

I would like to "upgrade" my database so that I can have my salesman have a copy of the data when he is out of the office. When he retruns I would like a two way Data Sync so data he entered will be added to my system and he will also get the latest data that is available. My database has about 70 tables and about 100 forms.

I am sure we will have to manage this with User names and Auto numbering to keep the primary keys "Primary" and I am not sure how we will need to manage to know what data is the most current. Maybe by a GUID or a time stamp. 
We would also need to have data conflict resolution... What happens if two people change the same record. How do we select the data we want to keep.