Expected Data Usage Compared to Drive Size
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.