From looking at this maybe I don't even need USG100 - looks like USG20 will do. The reason I started looking at USG100 is because at this Firewalls Comparison diagram (scroll down a bit):
it shows that only USG100 and higher support L2TP, which seems different from diagram you linked to. On diagram you linked to they all seem to support L2TP. Am I correct that USG20 and USG100 both support it?
said by Brano:
USG100 supports IPSec VPN, L2TP VPN (Windows VPN) and SSL VPN. Multiple simultaneous connections to the router are not a problem. If you have VPN client on LAN side of a router connecting to internet that's not a problem either.
However if you have a need to create VPN to the router and simultaneously another same kind of VPN to a LAN VPN server that's not possible.
Did you want to connect with the Cisco VPN client to USG? I'm not sure if that's going to work it may need to be tested.
It's fairly easy to setup, lots of how-to on this forum, quite good user guides too.
And you can always come back with specific problems here.
Maybe I didn't explain myself clear enough. I need to maintain 2 site-to-site VPN connections from USG*** to 2 other sites (one IPSec, another L2TP). I don't need other sites or clients connect to USG***. Can this be done?