NAS drive as backup

[epoxi]

I have been living dangerously for a while now since my data exceeded my external HDD and really should a proper backup solution. I am quite keen on buying a Synology NAS drive, people on HotUKDeals suggest buying a HP Microserver and running Synology's software on that, but I think I'd rather buy a ready-made offering from Synology and hide it in a cupboard somewhere so even if the house gets burgled I still have my data safe.

 

So my questions are as follows:

- Can I backup multiple computers to the NAS drive?

- Can I backup off-site computers to the NAS drive? (for instance could a family member elsewhere in the world back up their computer to my NAS drive instead of using a paid 'cloud backup' service?)

 

And a more complicated question:

How good is the security? For instance, I place a video on the drive from my computer, I want this to serve two purposes: the file is a backup of the one on my computer, but also I want to be able to stream that video to devices on my network. Let's say I get some ransomware on my PC, it goes through and encrypts all files .mp4 available to my PC: surely since the NAS drive is a network drive my PC has access to, the video on the NAS will get encrypted too, defeating the purpose of a backup?

Edited June 13, 2015 by epoxi