First: welcome to the forums!
Let me start with saying that the infrastructure of VBR and VBO are different. The repositories are not the same (VBR works with "flat" files while VBO has a running DB archive as a repository). The proxies have the same name, but they also operate differently. VBO proxies will do the work to connect to O365 and download/ backup the data for the users in the jobs that it manages.
I would suggest you deploy the VBO infrastructure as virtual. One of the advantages is that you can add more resources to it when necessary, but more importantly, that you will be able to use VBR to backup that server also. There is logic in VBR that will recognize it as a VBO server which means you could even do ILR from the VBR console in case it is needed.
For your requirements, I would suggest to start small with the VBO server with one or more repositories attached to it. Start with 200, then 500 and so on mailboxes and maybe you will need to deploy a second VM that will act as a proxy server (Based on the numbers you provide I doubt it, but on the other hand, when SharePoint and OneDrive data comes into play in the future, you might need to scale-out anyway)
The Exchange explorer is just a console and can be installed on the VBO server. But it can also be the one from VBR 9.5 U3, and you can even install it locally on your workstation. Nothing special needed for this one.
About JetBlue: Yes, it doesn't dedupe (running database
) but it delivers compression out of the box. You have 600 GB consumed today in O365, so you will not need 12 TB at all. VBO works like this: It backups the data the first time as a full backup. After that, only the changes are protected. Each repository will have a retention setting (default 3 years) which means that each item in your mail account, which is older than 3 years won't be in the backup anymore (a bit different from the retention than VBR). Read this here (as it is important): https://helpcenter.veeam.com/docs/vbo36 ... tml?ver=15
Hope I have answered most questions, if not, let me know