you want to make site and you have a design.It's not enough.
Also you need to find out which theme are best for your site and about maintenance.
Using Custom Design you can make your site Unique.
Custom Design gives you total control of your site.
But taking lot's of your time.
Also you need to pay for Developers.
Ready Made Theme
It saving your time.
Also helping to saving cost of hiring Developers.
Easy to design your site.
It provide a much more Functionality, and make site easy to use.
you need to use put your site content.