Introduction - how it works, why not self build?
The biggest determining factor for any project is budget.
I can do a five page brochure type site for £100. This would be based on one of my basic designs, similar to the site you are reading. Content would be added and the site would be fully operational.
Your only other option in this price range, would be one of the "do it yourself platforms" such as Squarespace or Wix. You would probably spend far more than £100 worth of your time setting the site up!
Next, how often do you need to update content? For example,I support a dentist site where they do not want to spend time updating information. They usually email me the text and I deal with it. this would not happen with a self build!
Finally, to keep this short,what if you want to add more complex functionality? The next step up is using a platform such as WordPress. Here you trade convenience for a complicated interface, for non techies. To sum up you need advice from someone who knows what they are doing and won't rip you off.
Wordpress theme modifications
I can modify your theme by using a child theme, this does not change your main theme and any modifications can be turned on and off.