Whatever helmet fits you well is what works. I have a schooling helmet with a cover so it matches my gear. As for vests, make sure it meets the safety standards. Ladies' IntecŪ Flex-Rider Body Protector* | Dover Saddlery
This one is rated and pretty cheap. I had an older model like this for breaking horses, the tail bone protection does make it difficult to sit deep, but you get used to it, and you should be light in the saddle on XC anyways.