See How to Breed RAREs section. They allow you to get items used to get RARE monsters. See How to Breed RAREs section. There really is no true best monster type. It all depends on the breeder.
Every monster has advantages and disadvantages.
This question can only be answered by yourself. I personnaly think it is worth double what I payed. But it is definately not for everyone. My best advice is to rent it first. Well you can't really beat it because you can always train more monsters.