The main premise of the movie made no sense, why does ultron want to kill the avengers specifically?

The Avengers are the only thing that have the capability to stop him, he knows all their strengths and weaknesses so it's important to stop/destroy them as soon as possible.

Why does Ultron act like stark at all if his code was from the alien tech?

The tech was the framework upon which the Ultron AI was built. Jarvis spends a great deal of time trying to integrate/interface with the base system. Once Ultron becomes sentient, he quickly absorbs a ridiculous amount of data from the computer systems in the Stark building, including surveillance footage and audio. Who knows how many petabytes of data that was?

Why is Jarvis suddenly on the same level as Ultron intellectually when Stark said they never thought theyd have AI in there lifetime?

Jarvis adapted, he probably learned a great deal from escaping being destroyed by Ultron at the start. Also, it's a movie universe with Gods, talking computers, magicians, space empires and the likes and you're being picky about how quickly a computer intelligence learns something new?

Why does hulk need a lullaby to regain control?

Because he trusts Black Widow and not himself? He knows she can calm him down which is the best option at this time. He is yet not in control to a point where he can de-Hulk anytime he wants.

When did they find out about wakanda?

The brand on the neck of the arms-dealer that Stark knew. They looked it up and found that the brand is used in Wakanda to mark thieves.    

Why did Ultron have "seismic" powers in the truck chase scene and why did he never use them again?

Fair enough, that particular power felt weird.

Why would the avengers start a fight with Ultron on a crowded road?

They had to stop him from getting away with the new body that would have been a complete defeat for the entire world had he gotten away with it or finished the upload. Priorities.

Why does a train track in the middle of a city have a dead end?

Maybe because it was the last station on a train line? Train tracks aren't big loops you know.

Why are the avengers still fighting amongst themselves when the hallucinations wear off, didn't they get enough out of that plot element in the last avengers?

Tony Stark just unleashed an AI that wants to destroy the world. It would be ridiculous if there wasn't tension after he went off on his own.. AGAIN.