AFAIK there isn't anything with that function to set the buyout price you prefer so you have to manually check everything and be there in the right time it's value is at $320 and do the whole process by yourself. Keeping money on any exchange site is not advisable so just send money to an exchange site at the moment your about to do the exchange and nothing else.
I think their are some exchanges where you can do this, but first your account should be funded with fiat currency and then you can place your buying price , when you price comes it will automatically executes the buying and you will be getting the price what you are expecting, but what other told keeping fiat currency in exchanges are not recommendable until you have full faith on that site or exchange.