Ok here's a small howto for the USB stuff I just wrote:
http://wiki.openwrt.org/HowToInstallOntoUsbDrive
It points to another small howto on usb drives... depending on how you want to work it.
OpenVPN is a fun little beast that takes a little while getting used to. I've set up a couple of different configurations only - but can probably help.
You will probably need to open up some ports on the router - I'm not sure if the scripts will do that for you automatically - and what ports will depend on what else you have to traverse/deal with at either ends.
For external ssh access you need to look in /etc/firewall.user - there's a sample on how to open up ssh.
What you need to know for OpenVPN:
Who needs access to what on either side?
Is there one computer each side - or is it a network?
Are you joining two networks on the same subnet?
What kind of keys do you want - pre-shared keys or certs? (Certs are always preferable)\
Are there static ip's or dynamic ip's? Who initiates contact?
Are there firewalls you need to traverse? Do you have control over them?