Definitely get a lawyer.
There is no such thing as a completely expunged record! Depending on what kind of a job you apply for later in life a background check will reveal this if convicted.You are not guilty of possession go to trial all it takes is one person on the jury that understands the drugs were not found on you and you're off free.Even if you get convicted(highly unlikely) this is a misdemeanor conviction,pretty minor but it could cause problems with future jobs.