The Liberty Train is probably a better choice if you don't want to fight back - you can armour it up and escape some pirates easily. You can find it on Baltimore Shipyards, New York.
Advanced Train can be found on Oder Shipyard, New Berlin.
Unlikely. Both Container transports (Kusari and Bretonian) don't seem to be for open sale. Plus, a Liberty corporate trader should aim to be using Liberty ships.
You can get away with the Kusari C-trans, it's a more or less open ship and has been for some time. Pending a viable alternative.
The Mastadon flies rather well, is tough as nails and its one rear firing turret is aces for knocking out trade lanes behind you. Though it's rather large.