A child theme is only necessary if you’ll be modifying core GeneratePress files or adding custom page templates.
If you’re not comfortable with coding, you probably won’t do either of those, so a child theme shouldn’t be necessary.
Any changes you make in the Customizer using GP Premium won’t be lost when you update.
You should be able to get your website looking very similar with GP – especially using Sections and a plugin like this for the content columns: https://wordpress.org/plugins/lightweight-grid-columns/
Let me know if you need more info 🙂