As you already know (if you don't know it yet, you better find out before it's too late) your website is your "face" on the internet, they way to present yourself (as person and/or as business) on the world wide web and it's going to say a lot about what kind of person are and what kind of business you run. If it's a good website it will definetly draw attention of your potential customers. If it's a bad website it will draw attention away from them. It can become a very important piece in your marketing campaign, so its design must be treated with a lot of attention.
When creating a new website you have 2 options: you can get a template from the internet (free or paid) for any of the current free CMS (like joomla, drupal, wordpress and all the others) or you can get some to design it from scratch. Each option has good and bad things, pros and cons that you should consider when choosing the best way for you.
Pros of custom website design
It reflects your ideas
All the existing free CMS on the market are limited in some way or another. Of course, you can add a lot of plugins, components and modules to customize the as you want, but, even so, the final version can be different that you intended it to be. It may not reflect your ideas about what you want to present on Internet.
Using a custom website design will have as result the best reflection for your ideas, your company and, why not, your brand. If designed professionaly the website will fit exactly in your maketing strategy on Internet and will have good effects in offline (what humans call "real life").
You can build it from scratch
The beauty with custom websites is that you start them from scratch and you can develop them in any direction you want. With the current free CMS you go in their intented direction and you modify it to fit your ideas - but that can be done in some limited ways. When big modification of any CMS are involved you can easily discover that you could spent the same amount of time, money and energy to create your own CMS with a much better result.
Consider that building a website from scratch will help you maintaining a strict timing of it's development: you can launch the website online with some well defined sections and options and you can always add new sections and option whenever you want or need them (after weeks, months or even years after the initial release).
Updates and alterations are easy to make (easy maintenance)
You can say that updates and alterations are also easy to make when using any of the current free CMS on the market. It's true, in some degree. But, as a webdesigner, I have a certain logic when I am coding. Every person has his own logic and his own way to write code and documentation and I say it's a lot easier to modify a piece of code that follows my logic that a code inside a CMS that follows other people's logic.
Of course, this would mean that, as a web designer, I should always be "in touch" with my customers and always be the person to modify the code. It real life it's not always the case, but the custom websites usually come with a strict documentation that help a people to follow their inner logic.
Cons of custom website design
It can cost a lot of time, energy and money
Depending of who you hire to design your website and what kind of information you want to put online the product can cost between 100 euros and 3,000-4,000 euros. Some big portals can cost more and it will take months to develop.
So, when you decide to have a custom website designed take into consideration what you want and how much time, energy and money you have to spent.
The updates and alterations may take some time
There are a lot of tools, some of them really good, that help someone to design custom websites. They can be used to update or alter the final product (the one you put online and want to modify), but in some situations there new sections and functions for your website will require the old-fashion coding - and that takes a time, a lot more that if you were using a free CMS.
Furthermore, you may need to hire someone to be always at your service when you need him to alter, update or solve problems with your website. If you have a small not important websites there are small chances for something bad to happen and you may not need any alterations or updates. Otherwise you will need money to pay for his services.
The final result may not be what you want
There is always a risk that the final result will not be want you wanted. In such situation you would spent a lot of time, money and energy for nothing - it can be a total loss and you may need to start designing the website all over again. It's worse when you realise that it's a faulty website after you have launched it online - beside the previous loses you can add loses in prestige and some bad words about you, your business or your brand.
Since working at Supravirtual I've seen or heard about a lot of situation with good and bad websites. We can testify (from our experience) that a website must be created in a professional manner if you want to yield good results. A professionally designed websites (with a lot of great content - we also take care of that for some of our customers) always drive traffic which can be converted to sales... and money.