We can discuss about it forever, but when it comes to protective clothing, helmets are the single most important item you can wear. And with everything, you should know a few things about them.
Any helmet bought in or outside Thailand needs a Thai or international safety mark, DOT, ECE205, or other. Chinese or Vietnamese copy helmets are illegal, not even to thinking of the level of protection they offer – likely a Thai plastic bucket will be better.
If you buy a 250,000 Baht motorcycle, it's best that you do yourself a favor and buy a helmet that is equally in quality. Yes, some of the helmets from around 2,000 Baht have international safety standards, but trust me, if you ride a bit faster than a Honda Wave 125 you need a bit more protection. And not forget that helmets have an expiry-date, normally 3 to 5 years – and because we live in the tropics the expiry-date is not to joke with, as excessive exposure to ultraviolet light can lead to damage to the plastic, meaning loss of protective properties of your helmet. Flip-up helmets may not always be as good as they claim. Although granted 'standards' in safety, a lot of flip-up helmets are tested as open face helmets. As a result, they cannot be relied upon to perform like a true full face helmet.
Eye protection isn't a legal requirement, but if you choose to use it in either glasses, goggle or visor form, then there are legal requirements, it's bullshit, but the law. They must conform to one of three grades of BS4110 for impact, scratch resistance and need to be transparent (clear visor).