Raysmes,
NAS boxes are great for backup. They are not so good as a file server.Some may be, but generally they are designed just as backup: low system memory, mediocre processor and big storage. If you can find one with a fast processor and large RAM (2Gb would be good rather than the 256MB most low-end ones seem to have) as well as Gb network rather than 10/100 Mb then you may have a solution.
File servers have a faster processor and larger RAM to deal with concurrent requests for files. The storage is expandable to whatever you want.
SME is designed as a File Server. And it provides VPN access either natively, via the server-manager or via the OpenVPN contrib.
I have struggled trying to turn a NAS into a file-server but the speed and the clumsy implementation of the software (it is firmware based) made it too difficult. Worked well enough when there were only 2 users but died in the arse when we got up to 5 users who were processing audio files and photographs.
SME does the job brilliantly - and a second SME running AFFA means we can be back up in minutes (20 or 30) rather than hours if the server dies.
In short, NAS is not the way to go. Stick with SME.
Cheers
Ian