Your DC's internet connection would all depend on the services and requirements of your customers needs.
If you are going to run a couple of low traffic hobby websites then you will not need such a big leased line... Whereas if you are offering an online backup solution to companies then you may require more bandwidth.
The best solution would be to find a leased line provider that will allow you to increase the speed quickly and effectively. If you have a line installed you will want a quick and easy way to upgrade that line when demand increases.