I agree with princess post, that base rock might not be a bad idea to look at. If your not worried about adding rock, thats not going to help out with the bio-load right away, then this will be the cheapest route to go. The rock will populate throug time and will then start to help out with bio-load.
other options would be buying uncured LR and curing it yourself in a rubbermaid container and then when finished add to your tank. This will help our with the bio-load right away in the tank.
IMO its not a bad thought to get base rock. Only problem you will have is moving all the rock in the tank, to get the base rock in and then topping it off with the other LR. Not a lot of work but will disturb the tank a bit but i dont think this will effect the tank much.