You do not have to buy LS, If you get southdown and some LR then the LR will seed your sandbed. It is best if you can get some LS but you really don't need a ton of it, maybe 20lbs or so, just be sure to get the good stuff from the ocean with lots of worms and stuff in it.
As far as a clean up crew, you will want to wait until after your tank has cycled. Then you get what you need to keep your tank clean. I like turbo snails and Mexican Trochus snails, and scarlet hermits, also if you have a sandbed you will want some ceirth or nassarius snails to live in the sandbed.