An Arada is certainly an excellent choice. Its a natural progression up from the Helian, with more than double the capabilities. Faster, more space, better stock weaponry. At a million credits its not cheap, but not too expensive either.
One note: an alternative choice to consider is the equally costly Freight-Courier (available at Crockett in the Thoso system). Its not as mobile as the Arada, but this is offset by the fact that it carries a surprising amount of space and upgrade room. Believe it or not, its about equal to the Zidara in overall weapon space (if you were to sell everything off both ships, the weapon space would be roughly the same). Its a solid all-around ship, and you don't have to make a special trip to the Crescent to get one.
As for missions, you get things rolling by visiting a UE bar and getting the mission UE shipping: Recruitment.
From there, you should align yourself with Frontier Express and/or Stellar Corp. Stellar Corp is easier to join with than FE, but FE missions pay better individually. They're also fairly dangerous, but you have a decent ship at your disposal. You do need a decent combat rating to get in (mostly harmless or the step above it, I believe), so blow away a few ships here and there to get it.
While shipping company missions pay better, do accept and finish UE shipping missions. Some pay well, and doing them raises your status with the UE. Eventually, when you reach a certain legal status & have a decent combat rating, you'll come across a mission that'll get you more involved with helping the UE.
At some point, get 6 or 7 jumps worth of fuel and head directly to the north of UE space. You'll eventually run into alien ships, and a bit past that is Miranu space. Find the Mira system, land on Mirava, and check the bar. The mission there is essential to opening up a huge portion of Override's scenario.
From there, good luck.
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