What kind of baby monitor should I buy?

It depends. It depends what insurance company you have and what monitor your baby need. Talk to your pediatrician. He/she might give you a better advice.
Not essential. Unless you live in a huge mansion, you will be able to hear your baby crying. Most new parents sleep in the same room as their newborn for the first few months anyway. A monitor usually just causes you to hear every little normal noise your baby makes and creates much sleep deprivation for the parents.