It doesn't hurt your score, if fact as long as you stay positive on it it helps your score. Student Loans are actually considered good debt but you have to stay on top of them and don't default, then it will destroy your credit report.
Who ever told you you would have problems getting a car if you have a student loan is very very an idiot. So pretty much the millions of people going to college cann't get a car loan? I think you know that is upsurd.
Also I noticed you mentioned co-signing, unless this is for your kids, DON'T not even for your husband or wife, I know that sounds bad but divorce is a reality. DON'T CO-SIGN. It's a bad move. Of course if it is for a child, then shop around first and only do it as a last resort.