Is this jacket rain resistant?

Yes , jacket is rain resistant.

what is the difference between the iconic racer jacket and the original iconic racer jacket which this is?

I purchased the Original Iconic Racer Jacket. The last one I had was a men's small, so I got a small in this one. It is way, way, way too small for me and I just found out the return policy is only for 30 days. I am not happy and wish I'd never bought it. Let the buyer beware.

I wear a 16 34/35 dress shirt 5’9 175 lbs Avg build. medium or large?

Based on my experience with sleeve length buying a L and XL, in your case I would recommend Large. Reason: I wear a 16.5 - 36 dress shirt, so according to the size chart I took Large. In reality the sleeves felt an inch too short, but were long enough on the next size up.