It is only necessary if your visa demands it and you are not covered by Medicare. However cover can remove some of the cost worries and can speed up some procedures for non life threatening ailments.
With whom to take cover is a matter of the tedious process of comparing many companies against cost and your own situation and requirements (including what you think you might require in the future). Look carefully at what is and what is not included, and what best matches what you want. I am afraid there is no quick answer, what is best for some is not best for others. (For me HIF is "best" because they were easiest to deal with by 'phone from England 10 years ago and have always been helpful.)
Net cost depends on your own tax position.