Also, keep in mind that serving temperature and fermentation temperature are two very different temperatures!!! You're not very likely to be able to use one fridge to do both - though if it's big enough, you can certainly cold crash in you kegerator / keezer.
A couple folks have cited $100 or so for a CO2 tank... Do yourself a favor, before you go and purchase a tank. Search your local area for a beverage supply place. Hell, ask the folks who gave you the kegs who they use for their beverage supplies... I found a place near me that offers a CO2 tank "rental". For a $50 deposit, they give me one of their 15lb CO2 cylinders, which costs $14 to fill. When it's empty, it's another $14 to swap out with another full one (I don't have to pay to have them recertified every few years, like I would if I owned my own tank). If I ever decide I don't want the tank any more, I take it back and get my $50 back. Not a bad option, and you may find something similar near you. I even lucked out and the guy hooked me up with with the ball-lock quick disconects and a gas manifold, free of charge.