Scalable and simple VPN solution for embedded devices

Scalable and simple VPN solution for embedded devices

I'm looking for some sort of VPN solution with the following requirements:
Server should be available for a Window Server 2008+
Client should be available for Debian Linux
Light-weight and scalable, easy to setup hundreds or thousands of clients
(devices)
Should be possible to query the server for a list active clients and their
addresses
The purpose of the VPN is mainly to be able to maintain lots of embedded
devices through SSH. The client devices are not used by people, they
perform other dedicated tasks. It will also be used to push data to the
clients when certain events occur.
Currently I'm using LogMeIn Hamachi which is working okay, but we are
looking for an alternative since we want to be able to manage the server
ourselves and run potentially thousands of clients/devices.
I have been looking into OpenVPN which could work, but it seems
troublesome to setup certificates for each client. It also seems to be
difficult to list active clients.
Does anybody know any good VPN solutions for this? Or different approaches?
Thanks