Networking/bandwidth/vlan recommendations for Skype for Business Online?

garryshape used Ask the Experts™
We're a small school and have free O365 education licenses. So we can use Skype for Business clients (S4b online) for peer to peer.  
We can also addon PBX and PSTN for $15/mo for users if we want, but we're currently still using an old on-site pbx system with old, antiquated phones.  
Whether we stick to peer to peer skype, or replace our phones, are there any references or guides or calculators we should look into for considering bandwidth and quality of service implications?    
We're about 250 employee and faculty.    
Just looking for recommendations/advice here, but I understand there's not always a once size fits all approach for everyone...
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