Hello,
Welcome to the forum.
1) have a look at www.ventrian.com, they have a private messages module
2) To be honest I'm not sure what the encryption is. I know that it uses unique machine keys for each installation which are then used to interpret the encrypted password. - Could be worth asking in the main dotnetnuke forum - let us know if you find the answer to that as I would be interested to learn more about that.
3) Have a look at the simple gallery at ventrian.com
4) Forum I use www.activemodules.com - For the rest a good place to look is
www.snowcovered.com where you can search through all of the developers various modules, you should be able to find what you need.
Hope this helps, thanks,
Lee