Side fan is important if you are using high performance GPU. though mainstream gpu already can produce a lot of heat, eg GTX 560Ti.
without side panel fans, the heat produced by your GPU will circulating inside the casing and will increase the ambient temp of your casing.

one side fan is already enough, either 1200/1600RPM, or 45 to 60CFM fans.
Case fan most ppl would recommend Arctic F12, cause its cheaper than other fans

Do be careful that depending on the rigidity of your chassis, a powerful side fan will cause unwanted vibrations and generate noise... further to that, side panels are normally not filtered so dust might enter easily from there straight to your GPU

Ppl here choose the F12 mainly because it has a good value/performance and CFM/Noise ratios.. they are not cheap per-se but rather affordable compared to other fans of similar class

Well that depends....
do you have an OC system? cause if you don't then the Hyper 212 can wait

IMHO its better to spend more on the chassis cause thats the main thing aside from the monitor that you'll be looking at most everytime you're using your PC... aside from that, a better quality chassis will also be sturdier and they tend to last longer... and also quieter I might add, especially if you intend to place fans on the side panel... a flimsy side panel will shake, rattle & vibrate causing unwanted noise on top of the fan's "whoosh" sound

a thing to note here is that, the more fans you add, the more noisy your system will be, dust is also an issue here and yes IMHO a better chassis will help with the temperatures