If you're going to be doing a site-to-site VPN, the traditional method is to use an IPSEC VPN. It's going to have better throughput, and since it's going to be router-to-router, you'll just have to set it and forget it. This is what I would recommend.
SSL-VPNs are better for mobile users. The throughput is lower, compared to IPSEC VPNs, but they're easier to configure, which is a plus for remote users. SSL-VPNs could be used for site-to-site VPNs, but that's not what they're really designed for.
At a minimum I would get a router running DD-WRT. Ideally, you should spend some money and get some higher end gear.
Radius is for authentication. It's not a VPN technology, so it's not really applicable to the problem.
We have a Juniper connected to a SonicWall. And we tried to get the SonicWall connected to a Cisco.
Sonicwall stuff is fussy, and they want to do things their way.