NOD32 is definitely super!! good interface and strong protection. If you connect to the internet permanently, be sure to use the 'Smart Security' version, not just the 'Anti-Virus' version.
About the USB HDD - take note the USB interface - 2.0 or 3.0 in your notebook - 3.0 is a lot faster!!!!
Yes, there are 2 versions. You can go to its official site and take a look:- www.eset.hk
Yes, they have such capacities, 500G and 1T, for some brands, even 1.5T, the size of the HDD may not be so compact. Ask the retailer.