I can't seem to figure out, when I wish to spend influence as imperial player, do I get to pick any agenda card I want to spend my influence on or do I have to draw x amount of agenda cards and then I choose which I want?

Also, there is an agenda card that rebels have a choice to spend 400 credits to get rid of or I get an extra agenda card, does in this scenario I draw from the pile and I keep the card or I have the option to buy the card?

http://www.iki.fi/a1bert/TheConsolidatedImperialAssaultRules.pdf

IMPERIAL UPGRADE STAGE
During a campaign, the Imperial player can purchase new cards during each Imperial Upgrade Stage. He can
first spend experience points to purchase new Class cards and then influence to purchase new Agenda cards.

• To purchase Class cards, the Imperial player examines all cards in his Class deck and may purchase as
many cards as he wishes by spending XP equal to the cards' costs. Once purchased, he retains the cards for
the remainder of the campaign.

• To purchase Agenda cards, the Imperial player shuffles the Agenda deck and secretly draws four of them.
He may purchase any of these cards by spending influence equal to the cards' costs.
━ The Imperial player does not show his drawn Agenda cards to the heroes. Any cards he does not
purchase are shuffled back into the deck without being revealed.

━ Most Agenda cards are immediately read aloud and resolved when purchased. The only exception is if
the card instructs the player to "keep this card secret." The Imperial player keeps the card and resolves
the effect when he plays it.

6 minutes ago, Bagzli said:

Also, there is an agenda card that rebels have a choice to spend 400 credits to get rid of or I get an extra agenda card, does in this scenario I draw from the pile and I keep the card or I have the option to buy the card?

You can purchase cards from the drawn agenda cards, and you can draw 5 instead of 4 during the imperial upgrade stage.