A 30g is a wonderful starter tank for saltwater fish, if you're uncomfortable or inexperienced. I myself em starting off with a 24g.
A 75g is another good choice. Something you'll typically notice about saltwater tanks, whether it is your first or your four hundredth, bigger is better.
The local fish stores (LFS) around here sell salt fairly cheap. 12.98 for 25gal. of Oceanic. It's is less costly when you buy in bulk, such as you did, but still for a small tank 30gal. will last you a pretty long time. For a 75gal. Not so much. 160gal. of salt should last you a long time in a 75gal. or a 30gal. Either way that's a good amount of salt.
If you decide to go with the 75gal. 50lbs. of live rock is enough. The "normal" ratio is 1lbs per 1gal. but you can fudge the number a bit, and if you want to get a full 75lbs. of rock just buy some base rock, which is significantly cheaper, and have the live rock seed it over a month and half or so.
Again, both the 30gal. and 75gal. are good for a starter. The 30gal. is a bit more risky because if something goes wrong the tank will pretty much go up, while a 75gal. can handle some mishaps here an there.
Overall, it's what you want to do and what you feel most comfortable with managing.
