My advice with bike PPE is go for middle price range if you are just starting out. In my opinion, if you go cheap you’ll end up paying twice. You do get what you pay for.
Sportsbikeshop is a great place to buy your kit if you have a store near you. If not, their delivery and returns policy is fantastic.
Infinity Motorcycles is a close second.
For a helmet, you need to determine not only the size of your head, but the shape. Certain helmet brands will suit your head shape which is good to know as depending on the info above it will narrow down your search criteria.
Good luck with the DAS