Is it acceptable for a researcher PhD or PostDoc to pay a freelancer to re-implement a Benchmark
This is a typical case of "ask your supervisor". He is the one who decides whether it is acceptable or not in your case.

A lot of research is carried out by multiple persons, and hiring someone is simply a case of adding an additional contributor to the research: there is nothing  wrong with that as long as you are honest and make it absolutely clear what your own work was and what was done by others. But, as said, your supervisor may not WANT to add another person to the research (many possible reasons for that exist), so ask him/her.