My general advice when purchasing Office 365 is that EVERYONE should consider E plans first. Why? They are simply more flexible and accommodate growth. For example, if you went with a P or M plan you would only receive Exchange Online Plan 1, which has a current limit of 25GB for email. Now let’s say that you need to have 26GB or email, your only option if you are on a P or M plan is to migrate the data out, purchase an E3 plan and then migrate the data back. Messy eh? Not to mention the transfer time of 25GB out of the M plan mailbox, 25GB back up to an E3 mailbox and then another 25GB down to an Outlook OST file. 75GB in transfers in total along just to scale up a mailbox not to mention any other costs!
It would have been much easier if you initially went with an E2 (and received a 25GB mailbox like the M plan) and then simply upgraded to an E3 plan. Doing so within the E plan stack would mean that all you need to to after purchasing the new E3 license is to go into that user and check all the E3 options and uncheck all the E2 options like so:
Once you have done that you simply press Save and the upgrade is complete. No migrating data, no data transfers, no interruptions, done, full stop. Once you have removed the E2 license you simply cancel it.
In many circumstances the other Office 365 plans work well but you must understand the limitations they provide, because in my experience most people bump up against them sooner or later. There are rumours that Microsoft will provide the ability to seamlessly upgrade from P to M and M to E plans BUT I stress here that to my knowledge that is NOT confirmed. I would therefore only be working the information on hand, which is, for the time being, each suite is stand alone.
So before you purchase ANY Office 365 plan I STRONGLY commend you to consider your growth needs and expectations. Even though the E plans are more expensive I honestly believe that are the best value given the flexibility they provide which no other plan can match.