Cokin ND grad filters have a reputation for having a colour cast, so I wouldn't recommend them. For your budget, I would go for the Cokin adaptors (Cokin P is fine for crop sensors, but I would suggest Cokin Z for full frame), but I would get a set of Hi-Tech grads instead. If you mainly shoot landscapes with a straight horizon, such as seascapes or plains, then a hard grad set would be best, but if you shoot mountains or hills mainly, then a set of soft grads are usually better. If you really want to save some money, then you can get away either a 2 and a 3 stop (hard or soft as appropriate) or one hard and one soft 3 stop. Don't forget to factor in the cost of appropriate adapters for each lens filter thread sze you own.