This is a very broad question but still i will give the best possible options at a very high level.
if your organization wants more advanced features such as exchange online, one drive,Skype for business, compliance features such as Data loss protection and audit logs along with SharePoint online then you need plan-3
else if you just need SharePoint online and with its own advanced features like BCS, auditing and compliance,excel online and info-path then you need Plan-2.
Now, Plan-1 will be least or stripped down version only limited to SharePoint online with above features. so if your organization and its users can live that then that's more than enough with only SharePoint online.
In any case, You need to integrate SharePoint online with your organization Active directory via Directory Sync services or you may configure ADFS as well. here the main point is you cant have two subscriptions synced the same active directory.it has some restrictions so go through the Microsoft documentation on the infrastructure needs before going in to purchasing part.
and finally, MSDN based office 365 developer subscription is ideal and fits well for the development purposes and to experience the ongoing roll outs. you can even integrate that with azure active directory (MSDN or your subscription) and have it used by multiple people. refer this excellent blog and i have implemented in my test tenant and provided access to multiple people. Reference Link