Here's some answers.

In Mission Beach, there are courts both on the beach and on the bay. Somtimes the bay-side courses are better protected from the wind. Generally, in California the wind starts picking up about noon, then dies down late afternoon. So I'd suggest playing in the morning or the evening - your girls will have more fun. Plus, it'll be a bit cooler.

There are typically permanent courts (i.e., poles buried in the sand), some of which will already have nets and some that won't. So, you can bring a net if you want, but you shouldn't need to bring poles. None of the courts are private, so if your girls are the first to get to a court, you have priority. Bringing your own ropes is a good idea.

The AVP ball is fine, just make sure it's the real ball and not the lower-priced version that is sometimes sold.

Go with bathing suits, not spandex. Nothing gives away a non-local more than playing beach volleyball in indoor spandex. Of course, if its cool, they can wear shorts and t-shirts over their bathing suits.