There needs to be a city with difficulty somewhere between Montego and the eastern half of the map. There's a huge leap there requiring several levels of ship upgrade (and the accompanying grind) to progress.
To get all of the "Buy Ships" achievements early, just buy and sell the same ship repeatedly. You get all of your money back and it counts toward the achievement.
Easy way to get the Hard Badge: Use the Remote Control, but make sure you don't run out of ammo. After you die, the T-Rex will continue to move along the screen, pushing your dead carcass along with it. It'll eventually push you past 2500.
 "Caribbean Admiral"){kind=icon}
 "Giants and Dwarves TD"){kind=icon}
 "Robot Legions"){kind=icon}
 "Nuclear Outrun"){kind=icon}
 "Red Remover"){kind=icon}
 "Back to Alien Party"){kind=icon}