Unless you absolutely cannot have this on your record for two years, do not get a deferred judgement. To answer your question though: No, you do not need to have a lawyer. You can ask the judge and present a short case of how you (ostensibly) have not gotten into trouble in the past, this was a mistake, blah blah blah. However, you're odds of getting the judgement deferred is higher if you have a lawyer present. Lawyer fees will probably be $500-1000 (ballpark).
Public Intoxication and Underage Possession of Alcohol are the only two crimes/charges in the state of Iowa that can be expunged from a criminal record. Play that to your advantage. Pay the fine, spend two years without getting into trouble (parking / traffic tickets do not count against you), and send in a form letter to the (Blank) County Court.
Since an individual only gets two deferred judgements, it tends to be wise to use them for a more serious charge should, God forbid, something else happen.
Don't sweat this too much. It sucks and it's embarrassing for now; but, in most instances, long-term (job, career) repercussions will be almost nil.