Does the military reimburse for prior paramedic training? Does becoming a paramedic before enlisting guarantee a job in the medical field?

You can receive Student Loan Repayment as part of your 3 or more year enlistment contract, if you took out a student loan for this, and that loan qualifies.
The paramedic cert does not guarantee a job in the military, but the training will help your line scores that qualify your for that job if it is available the quarter that you contract.