I'd add a couple questions to what's already been said.
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Will I be able to see the campaign events through my own Google Analytics account?
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If so, will you be setting up campaign goals with GA for me or is that something I have to track myself?
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What types of recommendations will you be making to improve the campaign?
One thing to add, I'd suggest that you set up the Google Tag Manager account and provide access to the agency. That way, you'll be able to have one Google code that addresses Google Analytics and GTM on your site. You'll also retain the historic information from the account if the relationship with the agency ends.