DSM-G600, DNS-3xx and NSA-220 Hack Forum

The reliability is going to be in the infrastructure your setup.

1) First, you will need a reliable Internet connection at the location where the DNS is going to be.
2) You will need an internet connection that allows incoming connections to be effectively established.
3) You will want to have a decent upload speed (outgoing) on the location the DNS is going to be.  For example, a typical 6 Mbps internet connections is asymetrical, it is 6 Mbps incoming but only 0.5 Mbps outgoing.  Some ISPs offer faser and some as slow as 364 Kbps.  Typically to get the same speed in and out, a commercial line is required, which is more expensive.  Not always, so shop around.  The outgoing speed is how you will be getting your data, so the internet connection where you are with your computer is not as important as the one where the DNS is located.
4) If there will be no one around to tend the DNS and your network, you will want to have the whole thing on at least 1 UPS in case the power goes out.  It will also prevent corruption due to a sudden removal of power.
5) I would reccommend setting up a VPN (Virtual Private Network).  It has the advantage of being very secure and easy to use, compared to other methods of accessing the DNS.  It also allows you access to the administrative pages of the unit and all other resources on your network.
6) You must also accept that there will be places that you may be where the internet connection at your computer will not allow you to access to your DNS (blocks ftp, VPN, etc.).  This is something you cannot control.  Also, be aware that bad things can happen that will not allow you access, such as a prolonged power outage or a circuit breaker blowing.  Heck, sometimes even hardware unexpectingly fails, but this is no different from any other device.
7) You will need reliable hardware for your network (router, switches, etc.).  Also make sure that no one else can physically mess with these component while you are away.

In short, the DNS is capable of what you want.  I have mine setup and I acess it when I'm halfway around the world on business.

It is possible, but there are some other security issues you would then have to deal with.  Obviously you would not people to VPN into your local network, but FTP on the other hand is not secure.  There are several options to do it in a secure method to protect your data and you network, yet maintain ease of use.  We can discuss them if you wish, but first formulate a very detailed scenario that includes who needs access to what, if different people need access to different material, but not the same material, who will need to upload data and to where, who will need to download data, how sensative the data is, how replaceable is the data, etc.  By doing this first, we can narrow down to the proper level of security, backup and ease of use.