Well if you are starting out are you sure you have a market for the product you plan on making?
You will be trying to take market share away from an established gravel pit operation, they will notice and react to your actions, have you though of that?
If you have a market to sell the product into at first why do you not rent a screen and put up an inventory pile of product and see if it actually does sell,
that way if the business is a bust (failure) you are not out the difference of what you comment your self to in cost of the screen plant and the amount you will receive if you are forced to sell it.
Do you have enough cash on hand to get the operation up and running, that inventory pile you plan on producing is going to cost you a whole lot of money to make, think of what it is going to cost you before you go forward.