it depends on the weight- as long as they are under 550kg, and I can't imagine one weighing more than that, then they are classed as 'quadracycles' and can be driven on a B1 category licence, any heavier and you need a car licence.
Some are legal for road use, and meet construction and use regulations, type approval, etc-most are not, and you'd really need to chack with the dealer. no, you don't have to wear a helmet but would be rather silly not to do so!!