What kind of equipment do you need to run a bag in box system?
it depends on how fancy you want to get, and how good you are at scrounging out the needed hardware.. new is very expensive but used can be very reasonable and sometimes free
for a basic system, you need:
a dispenser. this can be a full-on restaurant or convenience store type of dispenser, or a bar gun system, like a wunder bar. some dispensers have refrigeration systems to cool the soda and syrup, but most use a cold plate which requires clear ice to work properly.
a carbonator. this is a unit with a motor-driven pump and a pressure tank that mixes filtered tap water with c02 gas to make carbonated water. sometimes they are mounted inside a dispenser but usually are a seperate unit.
bib pumps. c02 gas-driven pumps that pull syrup out of the bib bags and pump it to the dispenser. you need one for each dispenser head or bar gun button you want to dispense.
bib connectors. fittings that connect to the valves on the bib bags, usually product-specific (coke and pepsi use different connectors)
co2 tank and gas regulators. you don't need a huge co2 tank, but 10lb is probably the minimum i would use... but you do need 2 regulators, one that can supply 100psi to the carbonator and one that can supply 40psi to the bib pumps. alternatively, if you want to save some co2 and already own an air compressor you can drive the bib pumps with air instead. you'll still need to run them at 40psi however..
tubing, fittings and clamps. how much and how many depends on your installation, but remember that the only metal allowed in contact with soda is stainless steel. copper and brass are not allowed and will create toxic water...
however...... if you are looking to dispense one flavor on an infrequent basis and want a simple system that can be added to an existing kegerator:
flomatic makes a fully-mechanical postmix dispenser valve that works well and requires no electrical power to operate. has 1/4" flare connections so it can be mounted to any surface, like the collar on a converted freezer. about $65 new.
a corny keg of force-carbonated water stored in a kegerator will last a while and is easy enough to refill when it runs out.
the syrup needs to be cooled, a 10 or 20 foot coil of 1/4" stainless tubing in a container of water in the kegerator should do the trick.
one bib pump for each dispenser valve, mounted inside or outside of the kegerator. best to have the syrup bib outside for easier pumping..
you'll still need 2 gas regulators, 70-100psi for the soda keg and 40psi for the bib pumps.