In regards to Klingons looking like Arabs in the original series. I don't think that was the intent at all.
Remember - this was pretty much at the height of the Cold War. And Communism to the West often was largely represented by Russians and Chinese. It was a short leap to make the reference to Mongols and Attila the Hun. If you look at some representations and images of peoples who populate Mongolia and other areas nearby, I think you'll see where some of the original look of the Klingons originated or was inspired by. But Roddenberry and the other creators were savvy enough to mix in other elements and make the Klingons more generic, I think.
It could also have been one of the last remaining vestiges in genre fiction of a racist trope later known as "the Yellow Peril". If you look at pulp serials from the early days, the trope saw expression in such characters as Ming the Merciless and Fu Manchu.
http://tvtropes.org/pmwiki/pmwiki.php/Main/YellowPerilThank goodness that Gene Roddenberry was consciously trying NOT to fall into those old tropes - I think I read somewhere that not only did he have to fight the TV execs to keep Uhuru as a character, but Sulu as well! If the original Klingons were "Space Mongols" that may have been one of the reasons why they were redefined later - because it was recognized that that had been a mistake.
Mind you - I'm making some informed conjectures here. I'm not 100% certain nor am I claiming to be. I'm offering my perspective based on experience and some casual historic research. Although I'm not old enough to have seen Star Trek when first broadcast - I did start watching it in the early 70s as a child when it was in syndication. So I more or less spent my earliest years growing up during that time. Enough to remember the perspectives of that era. For reference to the topic of "the Yellow Peril" trope, I can say that I read some early - EARLY - science fiction (I recall the original Buck Rogers novel was pretty blatant about it) and was curious enough about the topic later in life to look into it.
EDIT: I seem to be largely in agreement with Memory Alpha on the topic:
http://memory-alpha.wikia.com/wiki/Depicting_Klingons