Hopefully someone with more real experience will reply, but I offer these tidbits....
Yes, DynDNS or likewise will work with all the ways you'd want to connect. There is nothing about DNS to be incompatible with how you connect since it merely tells one device the IP Address of another device.
I own a pair of Linkstation Pro 1TB NAS units. So these are all I'm familiar with in the Buffalo line of products. I am using the FTP access for one share on one of the units. It seems to work well.
Both Windows and Linux offer the ability to "mount" or "map" an FTP share to the local filesystem.
I don't think (but also don't know) these units support secure FTP. Of course if you have the know-how and equipment, you can setup a VPN to your home network and use that to provide encryption.
In my humble opinion, these units are pretty weak when it comes to the Internet sharing capabilities, but it's nice to at least have the built-in FTP. Compare these to the Linksys router for example. With that, you can configure and require HTTPS (SSL) access, specify a custom port number.
With browsers supporting FTP natively and Operating systems supporting FTP in their mounting/mapping capabilities, there is a lot you can do with the basic FTP on the Buffalo.