This site runs on just,
- a single Raspberry PI 2 Model 2
- using a Samsung Evo 32GB microSD card
- powered by a 2 amp usb power supply
- and connected to a 'reasonable' Internet connection
The Samsung SD card was chosen, not because of it's class, but because of it's random read write performance as discussed by Raspberry Pi HQ. You may also want to check 'The Wirecutter' for an up-to date assessment of what currently may be the best microSD card to buy.