Cat s is structural damage causing a write off so much that the car has had to be reregistered with the dvla
Cat n is non structural damage such as lights but has still caused a write off but is not that bad so no need to reregister with the dvla
Don't touch these cars!
Choice is yours but i'd stay away.
There is because insurance for first time/young drivers is expensive anyway, if you buy a car that's previously been written off then the insurance will cost even more. Plus there's the cost of getting it evaluated by a firm.