IF YOU DO NOT HAVE A GOOD SET OF GAUGES DO NOT MESS WITH THE A/C. It is way too easy to over fill the system which can and probably will ruin the pump and then you are really in a hole! For $100 let a qualified shop check the system. evacuate and refill. It is not that much more than you messing around and buying all that you need from a store.
I know this from personal experience. The gauges that come on some of those store bought A/C charging kits are not accurate and will get into trouble.