As Snapper said - the large tank is just a convenience thing and CO2 consumption rate will vary depending on Ca/alk demand of the tank, efficiency of your Ca reactor, what media you use, etc. Also, whether you're topping off with just freshwater or kalkwasser can make a significant difference too. The Ca reactor doesn't have to be driven as hard to maintain Ca/alk when used in conjunction with kalkwasser.
Something else you want to consider before you get a tank is where you're going to get refills. Some places do a tank swap, so if that's the route you're going, no sense in buying a new tank. You'll just end-up with an old crappy one after your first refill (swap). If you're going to get your tank filled locally and you want a nice shiny tank - by all means, go for it.
FWIW, the regulator-to-tank connection should be flare thread and teflon tape should not be required (nor is it recommended).
HTH