I don't know of a NAS that does FTP, but I'm sure there's a custom linux solution out there on some commercial NAS if you dig enough.
If you go the NAS route, try and find one that also can do automatic backups to another device.
Personally NAS devices scare me when they aren't somehow backed up to a secondary NAS or *something*...
A lot can go wrong with a NAS device. You could have firmware or OS issues, the NIC can die or be blasted by a surge, an internal HDD could die, or the controller could bite it; then where are you?
Best case scenario it is a mirrored RAID NAS and you can take it apart and extract data from one of the drives. Assuming the file system isn't proprietary, then you're screwed. If it's a different RAID setup and the controller goes, then you're screwed.
So personally, I still recommend servers. Maybe I just haven't done enough research on newer NAS devices.
-------
But on another note, have you thought about how are they handling workstation backups, if at all?
A server may still be the best solution if they don't have a backup strategy, and you could score extra billable time setting up a service for them they didn't have before.
If they have less than 10 PCs in the office, I've used WHS in the past for situations like these. Cheaper than a full server OS, good replacement for NAS, and can keep all workstations backed up via the WHS connector software.
More than 10 PCs I've even seen MS recommend dual WHS boxes, but your newer more professional option is
Windows SBS 2011 Essentials which does the WHS style workstation backups.
If you're not concerned with workstation backups, then look into
Windows Storage Server 2008 R2 which I believe can be found on some...
you guessed it...
NAS devices. It's Windows, so your file system will be NTFS, always a plus for easy data recovery... and it's Windows, so you should be able to setup an FTP service on it. But do the homework before making that assumption if you're buying WSS preinstalled on a specialized NAS device.
Probably not on NAS devices, is
Windows Storage Server 2008 R2 Essentials which includes the WHS style workstation backup just like the standard Essentials server.