Been using OD for 5 years and remember starting out. I do the IT in the office and worked in Tech and was a computer hobbist for my teens so I'm comfortable with most simple computer tasks, incl. networking, building pcs, installing programs. Not sure what your IT / IT support setup looks like or how big/small your practice is, but we started with one server (OD is a pretty light weight solution) a desktop for the front desk and 2 laptops for the opertories and one as a patient kiosk.
So for our setup we hardly filled up a few GB for patient data. What takes more space are the images for Xrays, IOCs, etc. Then finally any scanned documents (using MFC printer/scanners) can eat up space quickly too, so best to get up and running on sheets and do your patient forms digitally. As the previous poster mentioned there is some variability on which devices you use and which folders you need to backup, so check with your vendors. If your using the OD imaging module (we switched over about a year ago then everything remains within OD and is well documented. Just need to make sure your devices support/have twain drivers.
It took us 4+ years (without cleaning up data) to get to 100s of GBs per backup. I started with manual copies of the mysql / opendentimages / <vendor specific directory> once a month at least to a portable 1TB SSD. Then added an online offsite encrypted backup (Backblaze) that backsup the whole server daily. Make sure to restore/test your backups as well or you never know if your doing it correctly and if the backups work. I tend to spend on business grade computers like from HP or Dell, but if your on a budget you find many older PCs from the same companies on eBay for a fraction of the cost. I'm using an i5 based EliteDesk 800 PC with 16gb. I swapped out the harddrives with name brand (Samsung/WD/Crucial) SSDs. 1TB is a lot for the main drive and having lots of free space helps reduce wear on the SSD. No moving parts increases the reliabilty of the system. But the nice thing about using these name brand PCs is that you can buy 2 identical machines and keep the 2nd for testing backups and as a hotswap replacement if anything catastrophic happens. I also got a 1TB usb ssd to copy any backups to (just remember HIPAA andake sure the drives are encrypted so that if someone tries to remove them they can't access the data without the key).
There are still somethings I can improve like incremental vs full backups to save backup time and space now that my backups are getting bigger. I could get a NAS for more backup storage, but make sure to also have a plan for offsite backups for the cases where the office might be damaged (e.g. fire or weather/nature related damage). Faster interfaces like 10GB USB 3.2 to reduce copying data off the server. But hopefully by the time you get to this point your practice will be doing well so that you can also look into managed IT support.
Last thing, try to use Windows server for the server from the get go. Windows Pro worked well for me till I had multiple desktops and laptops >10 accessing the server at the same time. While Pro can handle 20 similtaneous connections each client might create more than one connections.
Hope this helps. Good luck!