When you setup a nas for FTP you need to open up a port in your firewall. If this is going to be a security problem. You may want to use remote storage space. One that comes to mind is http://rapidshare.de/. This is a free service, with a option for PRO. I have been using this service for over a year now, works great. If you do not have the preimum service, files not downloaded in 30 days will be deleted. When yu upload a file thaey give you 2 links. One is to the file for downloading, the other to delete. With out the link you will not be able to retrive it unless it is in your folder files.
I use a Snap 2200 for my NAS, RAID 1. NO outside world contact.
If you are using Firefox, FireFTP is a good ftp extension.
Just as a personal recommendation, I'd stick with IE for FTP. Just to avoid the potential confusion/liability of having 3rd party people install software on their PCs. A lot of times, corporate IT departments won't let them install it, and lots of times people are just reluctant to do anything to their PC at all.
IE should work fine though, just send them a link in the format:
They can use drag/drop with IE just like Windows Explorer, so it's a lot easier for them to grasp.
As Blue68F100 said, you will need to open a port in your firewall for FTP. That could be1 a security concern depending on how you have your NAS security setup and how paranoid you are. There's no getting around that unfortunately, unless you wanted to setup some sort of VPN, but that's even harder to accomplish with users whos PCs you don't control.