If the answer to the following 3 questions is "yes", you should get a CTO. If answered "no" to at least 2 of them, you could probably manage with a freelancer or part time employee.
1. Is there a lot of (technological) work needed (the "Work")?
2. Is the Work ongoing (for at least 1 year)?
3. Do you need someone with a unique talent to do the Work (versus the fact that many programmers can do such work)?
Despite the above, if you're looking to raise capital (with Venture Capital firms), you should consider getting a CTO co-founder or full time employee as VC's are reluctant to fund startups/ventures that don't have a CTO as part of the team.
If you do decide to take on a CTO, make sure that you use a reverse-vesting mechanism (there's information on this online but I'm happy to help).
I've successfully helped over 300 entrepreneurs and I'd be happy to help you as well. Good luck