Since you mention DDNS, I assume you're hoping to access files on your home FTP server while your away from home?
I highly highly recommend you don't do this. FTP is an ancient protocol developed back when the Internet was young and naive. It does not encrypt anything - the username, password, and your files are all transferred in the clear. Anyone snooping any network between your location all the way to your home ISP can capture all this info. They could even use the username and password to login to your FTP server and grab all your files, delete them, store their own stuff, etc. It's a recipe for disaster.
The ideal way to do this is to get a router with a built-in VPN server. You set that up, then set up the VPN client on your laptop or whatever computer you plan to use while you're away from home. Once you connect the VPN client to the router's VPN server, that computer will see your home network like it was at your house (though the speed will depend on your home Internet connection). If it's an OpenVPN server, use tun, not tap.
Then you can simply view network shares on your home LAN from your remote location, as if the computer were on your home network. Instead of using your old Dell (which is probably a power hog and will cost you ~$100/yr in electricity to keep it powered on 24/7), just plug an external hard drive into the router. Pretty much every router with a VPN server also has a USB port for plugging in external drives. Then you can configure the router to shares the content of the HDD onto your home network. Which means you can also access it when you're connected via the VPN.