I understand the undergraduate Stafford Loan limit is $57,500. I took out a parent loan for my child's undergraduate education. Would this parent plus loan count towards the $57,500 cap if I decided to return to school to work on my own undergraduate education?
Your PLUS Loan Limit and Direct Loan Limits are independent.
Your PLUS Loan cap is the cost of attendance (sounds like your child is already done with school so this doesn't apply).
And you're correct the current undergraduate limit is $57,500.
However, be very cautious in taking out student loans later in life. It's the #1 way I see people getting in trouble going into retirement. At this point in time, if you can't afford to pay for your college and save for retirement, you might want to re-think your options.