I've got a full face Caberg bike helmet, always wear it on the Ring. I know most people don't but i prefer to wear one.
Rather than buying online completely blind, just go to a motorcycle clothing/helmet shop and try some on. Sizes differ from range to range and some are shaped differently in terms of where they put pressure on your head/cheeks to fit snugly, and I found some quite uncomfortable. Find a comfy one and buy it, or note down the brand/size and then get it online
