According to what you have written, you are a front-end student / developer, and for that reason, some points are very important to get ahead of you before you get bored with clients or your own code.
HTML is not a programming language - > Therefore it does not process on the server (such a machine that hosts your site). Therefore, it is not possible to create dynamic (of the type you want) sites with only html.
HTML5 is a programming language - > Can someone raise this topic. I recommend that none of you discuss this subject here, but in the chat, as it is extensive and may distort the topic. In short, version 5 of HTML can generate this debate, but even so, it does not serve to determine what you want. Stay away from this topic (in this topic).
Wordpress does not fit all cases - > There are customer profiles that require constant changes in platform appearance. Many of these changes may not cover the budget that was combined (do not fall for the 100% modifiable wordpress template). This varies greatly according to the purpose of each. If this project you took, is in parts to extend the front-end experience, I see no problem taking longer than the time you planned, and fix this problem in your next project. If this is not the case, just offer customer-ready templates, and explain that modifying them is more expensive and out-of-budget. Convince it's worth having your website up for a reasonable price in a reasonable amount of time, which is what Wordpress makes possible, because of its ease of installation. Before making the decision of Wordpress, ask a few questions in internet forums out there.
Study PHP - > I'm not here to tell you to become a PHP developer. Is not it! Is that PHP as many languages, has function of writing and reading on top of files. For a quick and inexpensive project, you can create a basic news system and your customer will be happy in life. Study if this is the case before setting up wordpress. Not to mention dynamic systems for web, it will hardly escape PHP. The fact of using Wordpress, you are already indirectly using PHP. Not to mention that is the ideal language for freelancers.