I think the phosphates might be your biggest concern right now. And caulerpa is cool at first because it grows so well, but that stuff is the *&%$#@ you hear people swearing about, when it takes over their beautiful tanks and live rock. Read some of the threads regarding removing every rock and picking them clean by hand.
Get it out now, and you'll be thankful later. If you want to keep some later (it apparently does have benefits, but I think it's more trouble than it's worth) you can let it go wild in a sump or refugium.
Clams are risky if you and your tank are still new - but they did seem to help my situation. SaltWater Connection is listed at the right.
A UV isn't a necessity (ask Chad), but again it sure helped me out.
A good skimmer is as close to a necessity as anything, for overall long-term water quality.