My best recommendation would be to just spend the money from the start on a good dual stage regulator, and run it on a 5lb CO2 tank since you have a relatively small system. There are threads on plantedtank.net that will detail the parts you need if you decide to "build" a system yourself, or you can just buy one pre-assembled from someone over there (new or used), or you can always pick one up from a company. FWIW...you get more bang for your buck going with either a pre-assembled regulator with all the parts you need than you would going with something sold my an online retailer.
The dual stage regulator will prevent a large amount of CO2 being dumped into the fish tank when the CO2 tank starts to get low (know as end of tank dump). At a minimum, I'd recommend picking up a decent regulator and hooking it up with a bubble counter and a solenoid; the bubble counter will allow you to dial in your flow rate, and you can put the solenoid on a timer so the CO2 only runs during your photoperiod.
There's a very, very wide range in pricing for a decent regulator setup. Needle valves can go for anywhere from 20-90 bucks depending upon the quality...which basically just makes adjusting the flow more precise. When I started out with my first CO2 planted tank, I went the cheap route and purchased a small regulator and cheap needle valve that I ran on a paintball tank. It worked ok...but adjusting the flow was imprecise and I did not have an option for a solenoid. I had issues with seals failing on the cheap regulators (800psi being controlled by a needle valve is a BAD idea), and eventually started an upgrade to a Concoa dual stage regulator with a solenoid.
You can find a Milwaukee regulator with bubble counter, solenoid and a decent needle valve in the $150 range if you shop around.
