Hi,
1) You can use OCS Standard edition to do audio and video conferences, no problem. You don't need exchange server to do this. The benefits that Exchange/Outlook give you are: presence/calendar integration & ability to schedule LiveMeetings through outlook appointments. If you don't use Exchange/Outlook to schedule, you'll probably need the OCS WebScheduler component.
2) you can do audio/video for external parties through VPN or WAN connections, no problem. You won't need any external DNS records in that case, as long as all participants are using your corporate DNS servers. If you wanted to include people who aren't a part of your corporate WAN, then you will need the OCS Edge Server and external DNS entries would also help.
Regards,
Matt