1. That really depends on the position of the game.
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If your peep is Level 1 it will give you a boost of most or all the level. As they get more skilled its effect is less noticeable. It depends on how much money you have. If you are just starting out 300 coins is rather expensive but after a while it's not much to pay.


2.
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We are assured that career upgrades do help but I can't say I've noticed much difference.


3. and 4.
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You don't get a choice, and are just assigned a random career/ like.
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To err is human; to arr is pirate.