Save up some more money, then think about starting a sw tank. Even buying good used equipment, $300 won't get you very far in the sw world. SW tanks are far more expensive to start than fw tanks.
Buying live rock alone will eat up most of that $300 (if you were able to get a deal somewhere for roughly 70 lbs at $4.00 / lb).
You wouldn't want to use the crushed coral, either -- there's a big sticky on it in one of these marine forums with more info.
While you're saving up your money, research all you can so you don't waste your money away on mistake items (an undersized skimmer, inadequate lighting, incompatible fish / inverts, etc.). It'll suck having to wait, but the more you learn before you start, the easier and less costly it will be.