Is it OK to get external help during your PhD if it is acknowledged?

I am starting a PhD, and an interested in getting technical skills I will need over the next few years.

Are there any ethical issues with paying for people to help you to get these skills, assuming they are fully acknowledged?

From what I have read, I am working with the assumption that it is OK, as long as they are not "contributing to the research". However, this seems a bit fuzzy to me. Where does the boundary between the acceptable and unacceptable input lie?