When looking at regulators, I highly recommend looking at a dual stage rather than a single stage. Dual stage will prevent the end of tank dump that can gas out your livestock.
I also highly recommend a solenoid and timer to shut the flow off when the lights are out. Both of these will add some cost, but in the long term, are well worth it.
To add some thoughts. Look on ebay for the Viktor VTS 250/253 series.
http://www.ebay.com/itm/Victor-VTS-...339?pt=LH_DefaultDomain_0&hash=item45feeddad3
This one has an oxygen fitting that would need to be changed to a CO2, but for the price compaired to a single stage, I think you will be much happier. When looking, also keep in mind that while a 5 lb CO2 tank will be in the 1000psi range on the supply side, the delivery is probably never going to go much past 30 psi. Your delivery side gauge will therefore want to be in the 0-100 or 0-60 psi range. You just have a tighter and more accurate control over something in the 0-200 range.