Get yourself a 4TB usable NAS device ( with room to add drives if needed later). That should be plenty for your situation and they are fairly inexpensive.
To attempt to answer your question:
For standard (legacy format) file backups of 200GB with a retention period such as yours, I would expect you to consume between 190GB and 240GB depending on the data change rate. How could it be less? Compression. We typically get ~20% compression rate overall. Most data never changes (pdf’s pictures, etc) , and if you are backing up QuickBooks, Word, Excel docs etc, you get a high compression rate and block level incrementals are small.
There are some data types (like app specific backups) that can generate a new GB+ file each day, such that keeping a month’s worth will consume more.
If you plan on doing local Image backups for six devices, you will consume a lot more space, but daily block incrementals tend to be small, and you can exclude the data folders from the image backup since you are already backing up the files separately.
Remember that with a once per month full image, you will always have two months worth of backups in storage since you can’t purge the previous month until the last block incremental for that month has reached 30 days old on day 59.

Thanks Steve. This is helpful.

You mentioned "...and you can exclude the data folders from the image backup since you are already backing up the files separately." What do you mean when you say this" I can't picture what type of files you are suggesting can be excluded.

You want an image backup of a machine to be able to restore the OS and all of the installed apps. If the users folder is not huge, then the image will also include a copy of all of the data files. But if the user data folders are large (greater than 200GB in our model) we exclude it from the image backups. The data is already being backed up separately via daily file backups with a 90 day retention period.
The image backups are for disaster recovery - not file recovery.
I would be happy to work with you to get things set up properly, let me know if you want help. (I don't charge anything)

Thank you for your explanation. Your approach looks like it has some real benefits, and we will keep your kind offer in mind!