Hey man,
Iv spent the last three months gathering parts and information about C02. Best thing is to research and read loads of threads on C02. This way you will become familure with parts and the best way to utilise the gas.
I myslef was clueless but iv set mine up for about 80 quid. That was including a good needle valve from swagelok, Regulator, Solenoid valve and atomic diffuser. All from ebay, all cheap and good quality. Im sourcing a fire extinguisher from a local company who will refill and service it for me.
Im delighted i took some time building and gaining knowledge over a bit of time instead of jumping in. Its great to have an understanding of why a C02 system benifits the aquarium, correlation with ferts and control of algae. Endless knowledge on this site alone.
Take time on Ebay because you can score some deadly deals with a little patience.
G'luk