Sorry for the delay...I was out all day! :-) You will only need to purchase the items you do not have already. However, because there is a built-in discount for starter kits, sometimes it is just as cheap or cheaper to purchase the starter kit rather than breaking out the items you will need.
The best way to determine the cheapest options is to call our customer service/order line at (888)775-6284.
Please let me know if you have any further questions! I am always happy to help!