I would never say that there is one "best" ERP solution for an ecommerce business as there are many factors to consider. Rather, I would encourage you to consider the specific needs you have in your business and then determine how you can leverage an accounting or ERP system to support these.

A couple of areas I would encourage you to consider are: the integration(s) you will require with shopping cart sites, inventory systems, dropshippers, shipping services etc. The types of products you are working with will also help to determine the type of inventory accounting requirements you are going to require.

As to whether an ERP solution is 'needed'...well, I can say from personal experience that I have seen many businesses significantly improve their efficiencies and profitability through implementing an appropriate solution. However, as with anything, spend the time either internally or with a consulting partner to determine the requirements your organization has. A good consulting and implementation partner can lead to great success of a ERP implementation, and a bad one can make you wish you had never taken the plunge.