MS Office / Hosted solutions / Office 365

We are looking to simplify/renew our entire IT and communications structure.

We currently have a file server, and a terminal server, but only 2 users due to downsizing, so our current system is more than overkill. MS Exchange is currently hosted.

Two users, but 3 PC's, as we have one in our meeting room. If I understand correctly, we would need to buy 3 version of MS Office, one for each PC. If we obtained hosting we would need only 2 licences, as the meeting room PC could be logged on as an additional PC by one of the two users.

While full hosting has it's attractions, I am wondering if it will bring disadvantages, eg. we currently use Outlook and Excel add-ons eg. for filing emails outside Outlook, and VT Accounts. I believe these add-ons would have to be hosted, too.

Would hosting restrict our ability to use MS Office add-ons, trial new software, create/amend our own Access databases, etc? I have contacted the company that hosts MS Exchange for us, but I am not really sure what questions to ask them. I have looked at Office 365, but I don't really understand that either.

Any tips or guidance would be greatly appreciated.