A dirt bike is bought under ones preference. This is because, before buying a dirt bike, you need to note the important factors such as the engine and the power. You need to know whether you want a two stroke or a four stroke engine or the one with the highest power. You also need to make sure that you get one that is comfortable for you. However, you need to make sure that you speak to an expert before buying one.
The size of the bike depends on how tall you are and how powerful you want it to be and the gear you should just ask when you go look at the bike but your gonna need quite a bit of stuff depending on what your gonna be doing