Right. So I would install a 12cm (120mm obviously) in the front blowing in. And I would install a 9cm (90mm obviously) in the back as an exhaust. Case fans vary in speeds and noise-levels - you don't need very high rpm case fans. Ones that blow 1200rpm to 2000rpm should be plenty of airflow. They can also be connected to your motherboard fan headers so that your motherboard can control the speed of the fan.
There are two ways case fans can be speed controlled by the motherboard. One way is voltage regulation which is the only way a 3-pin fan can be controlled. The other is PWM which requires a 4-pin fan and fan header. PWM is more expensive but allows the fan to spin slower than voltage-regulated fans making for a quiet system when at idle.
Fans also use a variety of different bearings. Regular "sleeve" bearings are very cheap and quiet at first, but wear out quickly. Ball-bearing fans are cheap, a little noisier, but last a long time. There are also all kinds of fancy oil pressure bearings and things like that available on the more expensive fans.
Since I think you are on a budget, I would just grab some decent ball-bearing 3-pin case fans and hook those to your motherboard's fan headers. The motherboard's CPU fan header should only be used for the CPU cooler's fan. Hook the case fans into the "System" or "Chassis" fan headers on the motherboard. Depending on your motherboard, you should be able to set up fan control profiles that will let you run your system more quietly while not under full load.