Even if you are going with fish only you still need LR, well YOU don't, but most fish do. If you have a tank that large you will want fish that you could keep in a tank of that size. They will need many lbs of very mature LR, which means that you will have to buy 100+lbs of LR mixed with base and let it mature.
please help with the cheapest way to do this
DON'T. Don't even bother upgrading. If you can afford the tank you have then stick with that.