The idea of purchasing and using a commercially available theme for the design of your website is sound and is the typical approach these days for many Websites as it saves a lot of time having to develop all the design code from scratch and gives you a number of useful, professional looking functions you can use that do not feature in the basic WordPress themes that come included with it or most free themes.
Generally, as one would expect, the demo of a commercial theme is created to demonstrate all the “bells and whistles” and functions it supports. Many hours of additional work goes into creating these demo websites to configure all of these features to show them off to their best effect. This of course looks great but what they don’t tell you is that each of these features needs to be configured yourself which can take a lot of time and effort and often requires more technical knowledge.
If you have some experience of design, know what you are looking for, and can look at the demo and its available features and visualise how it will look for your particular requirements then this can make the task a bit easier (unfortunately, unless you are working in the creative industry, most people find it very difficult to visualise what an actual design will look like). Then of course, you have to build it !
One other big problem is that in most cases, you need to purchase the theme and install it and experiment in order to see how to configure it – sometimes you find it doesn’t actually work as you envisaged or it contains bugs so you have to find another theme and start all over again. Clearly this can be expensive and takes time to do.
In addition there are literally thousands of “plugins” available that you can install into your WordPress installation to add extra functionality to your website, some are free, others you have to pay for. Like anything, some of these are better quality than others. Some work fine but others don’t and you can often find that they might work with one particular theme or plugin but not with others. This can be frustrating and time consuming – you often need to install and configure the plugin and try it to see if it works. When you find it doesn’t, you need to start again with another plugin and repeat the process, often multiple times.
Inevitably these days, you will also need to do some customisation to the design to put your brand on it. In some cases you will also need to customise the theme to add or remove some features (or maybe fix some bugs) so this adds to the amount of work you need to do. Unless you know what you are doing this will be something you will need the help of a web designer / developer on.
Bottom line – life just isn’t that easy I’m afraid – this is where you need good technical knowledge and experience or lots of spare time on your hands to experiment and learn.