Well since I am just putting the finishing touches on a keg fridge I might be able to give you some idea.
Yes you need a dedicated fridge of some type. Chest freezers work good also. Look around in the want ads you migt find one cheap.
I did a 2 tap set up on mine, got the fridge for free. Since it sounds like you can't be drilling holes in the fridge in the garage you'll need to go with picinic taps which are cheap. The most expensive item will be the CO2 tank and regulator. You can expect to pay around $80 for a 5lb tank and another $50 for a single guage regulator.
Altogether I spent around $125 for mine, but I got the tank, regulator and fridge for free.