Hi SadiqHaji,
In Office 365, we have multiple plans for the customer to meet different kinds of requirement. And generally, there are four basic services. Exchange Online is for the email services (We can all use the custom domain within the email address which includes this service.). Lync Online is for the IM and HD video conferencing. SharePoint Online for file storage and team collaboration. And the Office applications is for editing documents.
Several services are integrated in one plans with one account which you can find these plans via the following link:
https://products.office.com/en-us/business/compare-more-office-365-for-business-plans
Meanwhile, there are standalone plans for different services, for example, standalone Exchange Online service, standalone Office applications service.
Based on your description, you may want to check if the standalone Exchange Online is best suitable for your specific requirement.
http://office.microsoft.com/en-001/exchange/compare-microsoft-exchange-online-plans-FX103764022.aspx
And based on my experience, the best choice is based on your specific situation. I recommend you get a trial Office 365 Business plan via the following link to explorer the features first.
https://products.office.com/en-us/business/compare-more-office-365-for-business-plans
Regards,
Charles