Why not make your own PFSense firewall? Get a good little minibones PC and flash it with PFSense. At my home I have 1GB WAN from ISP and I use a barebones system by Jetway. Its been rock solid for 2 years straight.
I'm using but it is a fanless design and my system is in a closet with only a fan there and it runs at 54c (can handle 105c max) which is great considering its location and that this is in FL...
https://www.jetwaycomputer.com/JBC375F533.html
For your needs. I could go a step up or two on the CPU and increase the memory to help make things like external DOS attacks harder to take down your system due to increased specs etc... Plus with 100 PCs you will have a decent amount of traffic.
But to give you an idea. I run 2 websites and 4 application servers from home and my firewall CPU is currently sitting at 1% cpu usage...
P.S.
Only thing you need to be concerned about is WiFi. The drivers on this unit are not quite compatible with PFSense but thats not a big deal. Just dont use the wifi on this device and get Access Points instead.