1

If you're interested in the Gulf countries, then the obvious thing to do is to study Gulf Arabic...but I presume that's not an option for you.

Study MSA. It's more transferable.

2

I know people who started with Arabic and they all learned MSA.

3

I first studied Arabic in Saudi, "High" or "Formal" Arabic being recommended for reading/writing. Having fallen out of practice for many years I buffed up on my Arabic with an Iraqi teacher and found it hard going. Moroccan Arabic is gutteral, Egyptian is variable, Sudanese Arabic can be incomprehensible in some areas. In a nutshell, Arabic is highly variable everywhere. I would suggest that if you learn "standard" Arabic you should be understood through most of the ME and stand a decent chance of picking up their nuances in conversation.

Dave