A 5 lb should last about 6 mos on a 55g tank.
To save money on a regulator, you can get a welding one from Welding-Direct
Or from Welding Depot: $42.90
Then make your bubble counter/gas separator out of a soda bottle.
You will need a needle valve. The "FV 1/4" from STC valve costs only $7.37, and performs better than the ADA needle valve IME.
Personally, I like using a solenoid on a timer to shut off the CO2
at night. There is a machinery "junkyard" near my house where I buy high quality industrial solenoids for $3.95. If you live near such a place, that that may be a good option. If that is not an option for you, then STC makes the 2W025-1/4 for $27.50 which works well. If you want to save money though, you can simply leave the CO2
on all the time, and run an air-pump and air-stone at night to prevent CO2
Regardless, of what you choose, shop around. Once you know what you want, find the best price. It is easy to spend more money that you have to in setting up CO2
Oh yah, for diffusion, run the CO2
line into a Power Head. This may be the cheapest and best method available.