Greeting,
1. You can check using windows "Add remove programs" and see major version of sharepoint (2000, 2003 or 2007) . You can also use the "Central Administration" web , to see visually which version you have. The minor version can be resolved by reviewing "Schema" version in the database.
2. Yes you can. You can create a class that inherit from SPListItemEventReceiver and do some validation of you need. Other way is you can use WF (Windows Workflow Foundation) and setup some validation.
3. I think this is no problem with Sharepoint - but if you mean an OOTB features then I don't think you can do without help of development tools. A minimum SharePoint designer can do this.
Hope it helps you.
Regards,
Riwut Libinuko
-http://www.plasmedia.com-
please rate me, if you feel this was helpful
Best regards,
Riwut Libinuko
SharePoint Architect, Singapore
Microsoft MVP | SharePoint Server | Singapore | cakriwut@gmail.com
**Please let me know if you need more information**