I don’t have the answers to those questions but the 2 nonprofits I’ve seen do it have gone with Polycomm. If you’re looking to replace some hardware as well a VaR or one of their reps should be able to help you out.
I’ve been using Lync / Skype for Buisness for 5 years across our entire school. We transitioned from a PBX that was hosted by our telecom provider. We use both soft phone (S4B app in windows/mac and iOS) as well as Polycom hard phones.
If you have specific questions feel free to reach out to me and I’ll share what I know.
We have a seminary on our campus that uses Skype for Business for video conference calls and remote classroom connections. Both in my experience working with them and their opinion Skype for Business is not reliable enough to rely upon for office phone use. They went with a commercial VoIP solution for this very reason.
We are using Skype4B for our voip, and transitioned to it back in the Lync 2010 days. We have around 100 user accounts with an additional 45 common area phones. Most are using the softphone via a USB attached phone (Polycom CX300). The rest are using PoE Polycom CX600 phones, and the common area phones are CX500s. These are all older phones, you’ll want to go with modern phones (Audiocodes HD phones, Polycom VVX, Yealink, etc.)
It has been a very solid system for us, but can be somewhat complex to set up on-prem. As with most things Microsoft these days, you can weigh whether setting something up on-premises is even the route to go. Skype for Business online, and soon Teams, are quickly taking over. I’d recommend you talk with one of the partners about your options based on your environment and your needs. If you are looking for an integrated communications package with chat, phone, and tie in with other MS products, Skype4B has been great for us. This is not a knock against Skype4B as a voice system because it does great with voice, but if you are looking for more of a pure phone system, there might be better options. If I can help answer any other questions, let me know.