If you just want 802.11ac… keep your router and get an AP with 802.11ac. But, it will do you no good unless you have devices that will use the 5ghz. If you are buying a computer or components, they will always specify what wireless technology they utilize.
Which 802.11 standard you use has virtually nothing to do with security.
Would you say I would be appreciably more secure with a newer router which has AC rather than the router within the isp's gateway ?
simple answer is no you would not be more secure, like tomk says the standards are more to do with data transfer speeds than anything to do with security.
it's the encryption that counts for security, wep, wpa, wpa2.
all newer devices as far as i'm aware can use all of those and it's only very old hardware that might be limited to wep only, or wep and wpa.
It's just an option. If you have a router you like and works for you, and you want to be able to connect utilizing 802.11ac, you could just attach an Access Point with 802.11ac capabilities. Sometimes that is less expensive.
Remember, what is often referred to as a router… is most often more than just a router. I believe your "box" is a modem (communicates with the cable company), a router (routes data to the appropriate computer and handles DHCP), an Access Point (allows you to connect via radio waves or wifi), and also a switch (I think yours has probably 4 places to plug in an ethernet cable.) If you had a "stand alone" access point, it would provide the radio communications/wifi and would connect everything to your network via an ethernet cable between it and your router.
On my network in the office, I have a router, that also has a 4 port switch, but no radio - downstairs. I have an access point upstairs. I happen to have a 5 ft antennae on the roof attached to the access point because I have a wireless bridge/access point located in my shop nearly 1/4 mile away connected to another switch that lets me have internet access there. And it so happens I also have another access point (connected to the switch) located on the other side of the shop which allows me to have wifi at my home which is another 600 ft or so away (Cant pick up a decent signal directly from the big antenna at the office because there is a pond bank, a barn, a cistern house, a storage shed, and some trees between my house and the office.).
Anyhow, I haven't tried to price equipment lately. I'm just suggesting that you look at your options. You don't need to replace everything just to get a different radio signal. You can just get a different radio signal. By the same token, if you want to use 802.11ac and your laptop doesn't have that capability… you might not want to buy a new laptop. You can get a USB wifi adapter that receives 802.11ac for $50 or $60.
Yes, I understand about the router within the gateway which houses the modem. That is great news about the Wi-Fi adapter that receives 802.11ac for $50 or $60.
No. The adapter just plugs into the USB port on your computer. It's just a radio transceiver. You Operating system will probably automatically download the appropriate drivers and install.
The access point would need to be set up just like the wifi on your router. You'd need to give it a name and setup security.