These are fiddly bits of kit in my experience.
They don't actually mix the syrup and the water until it gets to the nozzle so if you're using a Cornelius Keg you'd have to have it pressurised with tap water and then the gun itself connected up to the boxes of postmix. There is a regulator that is usually fixed to the underside of the bar which controls the flow of the unmixed syrup and regulates the quantities for mixing. You'd need a managment board for the different syrups (coca-cola,diet coke,tonic etc) to push them through the gun and mix with the water.
All service and maintenance of these things is done by Coca-Cola themselves and they send a man round in full Coca-Cola kit to have a butchers at your gun if you knacker it. He does odd little tests for colour and taste until he's satisfied it's regulated the Coke way. They are expensive but most Bars and Pubs get them free of charge on the obvious stipulation they dispense Coca-Cola products. They almost always remain the property of the Coca-Cola company so unless someone has bought one from the Company then chances are it's a little suspect.
Easiest thing to do would be to mix the syrups and water into a Cornelius keg and run this through your cooler and out of a beer font on your bar or similar. I never experienced the commercial use of Cornelius Kegs but I'm guessing it was a similar use to this. Alternatively buy glass bottles!