SSHFS Server for Windows?

There are really no such thing as a special "SSHFS server". What SSHFS does it that it mounts a SFTP filesystem. In other words, what you are looking for is a SFTP capable SSH server for Windows. There are a bunch of SFTP capable SSH servers for Windows, but I don't have enough actual experience with any of them to actually make a recommendation on which to choose.

Any good SSH server implementation should do the trick.

I use Cygwin on many Windows machines particularly for its SSH+SFTP/SCP services (though I've never tried to connect to them using sshfs, it is a port of the same OpenSSH that most Linux distributions include so I expect it would work just as well).

CopSSH is an option I've seen spoken of positively too. I've not used it myself but it is essentially just OpenSSH (and related packages) from Cygwin without anything else so may be preferable if you want a minimal setup.