Is it advisable to buy the service of an SEO expert?

It’s very easy to avoid violating Google’s guidelines if you simply take time to read them. They are really very helpful and fully explain what you should do to improve your chances of being found by search engines, and what not to do, to avoid incurring penalties.

There is a helpful beginners guide: “Search Engine Optimization Starter Guide” and more detailed information for those who want it: https://support.google.com/webmasters/topic/6001981?hl=en&ref_topic=3309300.