If your ok with doing weekly water changes and having a low fish load, and not having anything wich needs great water quality and have plenty of live rock, then no it's not needed... otherwise, yes, you need it.
IMO, even with weekly water changes, it is still extremely beneficial
you can get by without it, but it is much easier and better for your tank to have one
just search for water changes as a key word here on AC and ull find lots of great threasd on that to help. as for the skimmers they are always great and id never set up a salt tank without one. they can range from $40-$300