Windows are good because with them you can see the inside of your case, which looks awesome, especially with LEDs, which you can easily get with your fans.
I would go with extra fans. They're not too essential, but you might as well. I personally own
these fans which are a really good price per fan. I haven't had any problems with mine so far and I've had them for almost a year. They are great filler fans if you have extra fan slots. Also, I found out that the case can actually support a total of 5 140mm fans and 1 120mm fan or 4 140mm fans and 3 120mm fans including the fans that come with the case. You can do either configuration, it doesn't make too much of a difference.
Here's a list of fans you can get:
140mm fans
120mm fans