Disconnections from wireless can happen for many reasons the 2 biggest I have seen are compatibility and interference(because 2.4 is so overused it happens, even running a microwave can cause issues).
I once had a WRT54G from Linksys and it just did not get along with my old notebook I would disconnect all the time when doing file transfers. While these kinds of issues are more rare, they may still happen with some setups.
If you happen to have more devices on wireless that also effects the overall bandwidth as well.
Your wireless speed should not pose an issue for the CCTV camera setup(you are still faster than your internet connection by the look of things), but if they CCTV cameras are always recording to the wireless system that is going to effect your bandwidth for backing up(it may even cause cut outs in the recording). For a test you can always run a wire to the system temporarily to see how it works for you.
Everyone's setup will be different so you generally have to find out what works best for you.
Onto the dual band. Dual band allows you to use 5ghz or 2.4 or both at once to get better speed as well as less interference. The 5ghz band has less interference because it is not used as much(this is the same reason most cordless phones(even cell phones tend to run a lower frequencies) have moved to DECT 6.0[dect 6.0 runs at 1.9ghz and is only called 6.0 because customers would never buy a 1.9 ghz phone when clearly the large number 2.4 and 5 must be better
]). Either way if the 2.4 ghz connection gets to much interference the 5 will just keep on going(or of it happens the opposite way, but it is more rare). Now in these cases you are looking at a more reliable connection. Top speed still depends on many factors.
Now you may have another option that does not require running wires and has a more constant speed, but still not as fast as gigabit lan. Homeplug based products work on your homes electrical wiring. They have more consistent speeds, but are still not perfect. This is just the next best option for a system that is not mobile and difficult to get a normal network cable to.
These devices generally have one or more Ethernet ports on each side and are not seen by your computer. Your system just sees a Ethernet cable at the highest supported speed all the time.
Sorry for the wall of text, but you have limitless options for connecting computers and devices together.
EDIT, your file sizes(10 gigabytes) should be optimal for network transfer.