Is one rear fan enough for a basic build?

You don't have to get a more expensive case for the extra fan. I'd say at least two fans, so that there's good airflow going through the case.

Although, looking at the NZXT and HAF 912 case, there's a $20 difference. If you want extra fans for the NZXT, you can buy two $10 120mm fans, or four cheap rosewill $5 120mm fans and the price will come up to the same as the HAF.
 
do what i do, have intake fans on the front on the computer to suck air into the case, then put a fan on the back of the case to push the air out which creates like a breeze or airflow which will allow the hot air to be removed quickly and the new cooler air pulled in. i noticed my temperature is around 10 degrees less using this method.
 
The low speed Rosewill fans would be an excellent choice for this build. The selected parts do not produce a lot of heat. If this were a high end build, overclocked as well, it might need more and/or faster fans. As described, it does not.
 
Thats true ..... depends on how much heat you generating of course but why risk premature component death .. You could add a fan to the side panel location for more intake and have a positive pressure set up and or get one of the 140mm for 120 mm location fan that ..... ( I think scythe makes ) but then your over what ya would have paid for a slightly more expensive case .. " decisions decisions "