set up VPN router as a VPN server and more

I have a 1 Gbps fiber-optic Internet connection from Cox (the “modem”), a Dell R710 server, and a home network using a Netgear Nighthawk X10 AD7200 Smart Wi-Fi Router (R9000). I would like up to 100 students to be able to access the server at my home, but protect my home network. Almost all of the students are running Windows 10. Because the Nighthawk router is in my family room, I cannot put the server there, but I can put the server near the modem. I’m thinking of buying a VPN router and connecting this to the modem, the Nighthawk router, and the server, setting up a VLAN on the VPN server and having students VPN from their school into the VPN router and only access the server keeping my home network safe. I would also like them to be able to access the Internet from the VPN router as their school filters sites that we need access to for this class. This VPN router does not need Wi-Fi. Once a week the students will upload a few 5 GB files, and then at other times up to 100 students will access a website on the server for hours at a time. I’m open to spending $200 - $500 on a VPN router, and I’m very concerned regarding performance. I’m open to trying DD-WRT or anything else. I’m guessing I’ll also need to use DynDNS. What’s the best way forward? Best configuration? Best equipment? Best VPN software for my needs?